/* * Fast Userspace Mutexes (which I call "Futexes!"). * (C) Rusty Russell, IBM 2002 * * Generalized futexes, futex requeueing, misc fixes by Ingo Molnar * (C) Copyright 2003 Red Hat Inc, All Rights Reserved * * Removed page pinning, fix privately mapped COW pages and other cleanups * (C) Copyright 2003, 2004 Jamie Lokier * * Robust futex support started by Ingo Molnar * (C) Copyright 2006 Red Hat Inc, All Rights Reserved * Thanks to Thomas Gleixner for suggestions, analysis and fixes. * * PI-futex support started by Ingo Molnar and Thomas Gleixner * Copyright (C) 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com> * Copyright (C) 2006 Timesys Corp., Thomas Gleixner <tglx@timesys.com> * * PRIVATE futexes by Eric Dumazet * Copyright (C) 2007 Eric Dumazet <dada1@cosmosbay.com> * * Requeue-PI support by Darren Hart <dvhltc@us.ibm.com> * Copyright (C) IBM Corporation, 2009 * Thanks to Thomas Gleixner for conceptual design and careful reviews. * * Thanks to Ben LaHaise for yelling "hashed waitqueues" loudly * enough at me, Linus for the original (flawed) idea, Matthew * Kirkwood for proof-of-concept implementation. * * "The futexes are also cursed." * "But they come in a choice of three flavours!" * * This program is free software; you can redistribute it and/or modify * it under the terms of the GNU General Public License as published by * the Free Software Foundation; either version 2 of the License, or * (at your option) any later version. * * This program is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU General Public License for more details. * * You should have received a copy of the GNU General Public License * along with this program; if not, write to the Free Software * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA */ #include <linux/slab.h> #include <linux/poll.h> #include <linux/fs.h> #include <linux/file.h> #include <linux/jhash.h> #include <linux/init.h> #include <linux/futex.h> #include <linux/mount.h> #include <linux/pagemap.h> #include <linux/syscalls.h> #include <linux/signal.h> #include <linux/export.h> #include <linux/magic.h> #include <linux/pid.h> #include <linux/nsproxy.h> #include <linux/ptrace.h> #include <asm/futex.h> #include "rtmutex_common.h" int __read_mostly futex_cmpxchg_enabled; #define FUTEX_HASHBITS (CONFIG_BASE_SMALL ? 4 : 8) /* * Futex flags used to encode options to functions and preserve them across * restarts. */ #define FLAGS_SHARED 0x01 #define FLAGS_CLOCKRT 0x02 #define FLAGS_HAS_TIMEOUT 0x04 /* * Priority Inheritance state: */ struct futex_pi_state { /* * list of 'owned' pi_state instances - these have to be * cleaned up in do_exit() if the task exits prematurely: */ struct list_head list; /* * The PI object: */ struct rt_mutex pi_mutex; struct task_struct *owner; atomic_t refcount; union futex_key key; }; /** * struct futex_q - The hashed futex queue entry, one per waiting task * @list: priority-sorted list of tasks waiting on this futex * @task: the task waiting on the futex * @lock_ptr: the hash bucket lock * @key: the key the futex is hashed on * @pi_state: optional priority inheritance state * @rt_waiter: rt_waiter storage for use with requeue_pi * @requeue_pi_key: the requeue_pi target futex key * @bitset: bitset for the optional bitmasked wakeup * * We use this hashed waitqueue, instead of a normal wait_queue_t, so * we can wake only the relevant ones (hashed queues may be shared). * * A futex_q has a woken state, just like tasks have TASK_RUNNING. * It is considered woken when plist_node_empty(&q->list) || q->lock_ptr == 0. * The order of wakeup is always to make the first condition true, then * the second. * * PI futexes are typically woken before they are removed from the hash list via * the rt_mutex code. See unqueue_me_pi(). */ struct futex_q { struct plist_node list; struct task_struct *task; spinlock_t *lock_ptr; union futex_key key; struct futex_pi_state *pi_state; struct rt_mutex_waiter *rt_waiter; union futex_key *requeue_pi_key; u32 bitset; }; static const struct futex_q futex_q_init = { /* list gets initialized in queue_me()*/ .key = FUTEX_KEY_INIT, .bitset = FUTEX_BITSET_MATCH_ANY }; /* * Hash buckets are shared by all the futex_keys that hash to the same * location. Each key may have multiple futex_q structures, one for each task * waiting on a futex. */ struct futex_hash_bucket { spinlock_t lock; struct plist_head chain; }; static struct futex_hash_bucket futex_queues[1<<FUTEX_HASHBITS]; /* * We hash on the keys returned from get_futex_key (see below). */ static struct futex_hash_bucket *hash_futex(union futex_key *key) { u32 hash = jhash2((u32*)&key->both.word, (sizeof(key->both.word)+sizeof(key->both.ptr))/4, key->both.offset); return &futex_queues[hash & ((1 << FUTEX_HASHBITS)-1)]; } /* * Return 1 if two futex_keys are equal, 0 otherwise. */ static inline int match_futex(union futex_key *key1, union futex_key *key2) { return (key1 && key2 && key1->both.word == key2->both.word && key1->both.ptr == key2->both.ptr && key1->both.offset == key2->both.offset); } /* * Take a reference to the resource addressed by a key. * Can be called while holding spinlocks. * */ static void get_futex_key_refs(union futex_key *key) { if (!key->both.ptr) return; switch (key->both.offset & (FUT_OFF_INODE|FUT_OFF_MMSHARED)) { case FUT_OFF_INODE: ihold(key->shared.inode); break; case FUT_OFF_MMSHARED: atomic_inc(&key->private.mm->mm_count); break; } } /* * Drop a reference to the resource addressed by a key. * The hash bucket spinlock must not be held. */ static void drop_futex_key_refs(union futex_key *key) { if (!key->both.ptr) { /* If we're here then we tried to put a key we failed to get */ WARN_ON_ONCE(1); return; } switch (key->both.offset & (FUT_OFF_INODE|FUT_OFF_MMSHARED)) { case FUT_OFF_INODE: iput(key->shared.inode); break; case FUT_OFF_MMSHARED: mmdrop(key->private.mm); break; } } /** * get_futex_key() - Get parameters which are the keys for a futex * @uaddr: virtual address of the futex * @fshared: 0 for a PROCESS_PRIVATE futex, 1 for PROCESS_SHARED * @key: address where result is stored. * @rw: mapping needs to be read/write (values: VERIFY_READ, * VERIFY_WRITE) * * Returns a negative error code or 0 * The key words are stored in *key on success. * * For shared mappings, it's (page->index, vma->vm_file->f_path.dentry->d_inode, * offset_within_page). For private mappings, it's (uaddr, current->mm). * We can usually work out the index without swapping in the page. * * lock_page() might sleep, the caller should not hold a spinlock. */ static int get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key, int rw) { unsigned long address = (unsigned long)uaddr; struct mm_struct *mm = current->mm; struct page *page, *page_head; int err, ro = 0; /* * The futex address must be "naturally" aligned. */ key->both.offset = address % PAGE_SIZE; if (unlikely((address % sizeof(u32)) != 0)) return -EINVAL; address -= key->both.offset; /* * PROCESS_PRIVATE futexes are fast. * As the mm cannot disappear under us and the 'key' only needs * virtual address, we dont even have to find the underlying vma. * Note : We do have to check 'uaddr' is a valid user address, * but access_ok() should be faster than find_vma() */ if (!fshared) { if (unlikely(!access_ok(VERIFY_WRITE, uaddr, sizeof(u32)))) return -EFAULT; key->private.mm = mm; key->private.address = address; get_futex_key_refs(key); return 0; } again: err = get_user_pages_fast(address, 1, 1, &page); /* * If write access is not required (eg. FUTEX_WAIT), try * and get read-only access. */ if (err == -EFAULT && rw == VERIFY_READ) { err = get_user_pages_fast(address, 1, 0, &page); ro = 1; } if (err < 0) return err; else err = 0; #ifdef CONFIG_TRANSPARENT_HUGEPAGE page_head = page; if (unlikely(PageTail(page))) { put_page(page); /* serialize against __split_huge_page_splitting() */ local_irq_disable(); if (likely(__get_user_pages_fast(address, 1, 1, &page) == 1)) { page_head = compound_head(page); /* * page_head is valid pointer but we must pin * it before taking the PG_lock and/or * PG_compound_lock. The moment we re-enable * irqs __split_huge_page_splitting() can * return and the head page can be freed from * under us. We can't take the PG_lock and/or * PG_compound_lock on a page that could be * freed from under us. */ if (page != page_head) { get_page(page_head); put_page(page); } local_irq_enable(); } else { local_irq_enable(); goto again; } } #else page_head = compound_head(page); if (page != page_head) { get_page(page_head); put_page(page); } #endif lock_page(page_head); /* * If page_head->mapping is NULL, then it cannot be a PageAnon * page; but it might be the ZERO_PAGE or in the gate area or * in a special mapping (all cases which we are happy to fail); * or it may have been a good file page when get_user_pages_fast * found it, but truncated or holepunched or subjected to * invalidate_complete_page2 before we got the page lock (also * cases which we are happy to fail). And we hold a reference, * so refcount care in invalidate_complete_page's remove_mapping * prevents drop_caches from setting mapping to NULL beneath us. * * The case we do have to guard against is when memory pressure made * shmem_writepage move it from filecache to swapcache beneath us: * an unlikely race, but we do need to retry for page_head->mapping. */ if (!page_head->mapping) { int shmem_swizzled = PageSwapCache(page_head); unlock_page(page_head); put_page(page_head); if (shmem_swizzled) goto again; return -EFAULT; } /* * Private mappings are handled in a simple way. * * NOTE: When userspace waits on a MAP_SHARED mapping, even if * it's a read-only handle, it's expected that futexes attach to * the object not the particular process. */ if (PageAnon(page_head)) { /* * A RO anonymous page will never change and thus doesn't make * sense for futex operations. */ if (ro) { err = -EFAULT; goto out; } key->both.offset |= FUT_OFF_MMSHARED; /* ref taken on mm */ key->private.mm = mm; key->private.address = address; } else { key->both.offset |= FUT_OFF_INODE; /* inode-based key */ key->shared.inode = page_head->mapping->host; key->shared.pgoff = page_head->index; } get_futex_key_refs(key); out: unlock_page(page_head); put_page(page_head); return err; } static inline void put_futex_key(union futex_key *key) { drop_futex_key_refs(key); } /** * fault_in_user_writeable() - Fault in user address and verify RW access * @uaddr: pointer to faulting user space address * * Slow path to fixup the fault we just took in the atomic write * access to @uaddr. * * We have no generic implementation of a non-destructive write to the * user address. We know that we faulted in the atomic pagefault * disabled section so we can as well avoid the #PF overhead by * calling get_user_pages() right away. */ static int fault_in_user_writeable(u32 __user *uaddr) { struct mm_struct *mm = current->mm; int ret; down_read(&mm->mmap_sem); ret = fixup_user_fault(current, mm, (unsigned long)uaddr, FAULT_FLAG_WRITE); up_read(&mm->mmap_sem); return ret < 0 ? ret : 0; } /** * futex_top_waiter() - Return the highest priority waiter on a futex * @hb: the hash bucket the futex_q's reside in * @key: the futex key (to distinguish it from other futex futex_q's) * * Must be called with the hb lock held. */ static struct futex_q *futex_top_waiter(struct futex_hash_bucket *hb, union futex_key *key) { struct futex_q *this; plist_for_each_entry(this, &hb->chain, list) { if (match_futex(&this->key, key)) return this; } return NULL; } static int cmpxchg_futex_value_locked(u32 *curval, u32 __user *uaddr, u32 uval, u32 newval) { int ret; pagefault_disable(); ret = futex_atomic_cmpxchg_inatomic(curval, uaddr, uval, newval); pagefault_enable(); return ret; } static int get_futex_value_locked(u32 *dest, u32 __user *from) { int ret; pagefault_disable(); ret = __copy_from_user_inatomic(dest, from, sizeof(u32)); pagefault_enable(); return ret ? -EFAULT : 0; } /* * PI code: */ static int refill_pi_state_cache(void) { struct futex_pi_state *pi_state; if (likely(current->pi_state_cache)) return 0; pi_state = kzalloc(sizeof(*pi_state), GFP_KERNEL); if (!pi_state) return -ENOMEM; INIT_LIST_HEAD(&pi_state->list); /* pi_mutex gets initialized later */ pi_state->owner = NULL; atomic_set(&pi_state->refcount, 1); pi_state->key = FUTEX_KEY_INIT; current->pi_state_cache = pi_state; return 0; } static struct futex_pi_state * alloc_pi_state(void) { struct futex_pi_state *pi_state = current->pi_state_cache; WARN_ON(!pi_state); current->pi_state_cache = NULL; return pi_state; } static void free_pi_state(struct futex_pi_state *pi_state) { if (!atomic_dec_and_test(&pi_state->refcount)) return; /* * If pi_state->owner is NULL, the owner is most probably dying * and has cleaned up the pi_state already */ if (pi_state->owner) { raw_spin_lock_irq(&pi_state->owner->pi_lock); list_del_init(&pi_state->list); raw_spin_unlock_irq(&pi_state->owner->pi_lock); rt_mutex_proxy_unlock(&pi_state->pi_mutex, pi_state->owner); } if (current->pi_state_cache) kfree(pi_state); else { /* * pi_state->list is already empty. * clear pi_state->owner. * refcount is at 0 - put it back to 1. */ pi_state->owner = NULL; atomic_set(&pi_state->refcount, 1); current->pi_state_cache = pi_state; } } /* * Look up the task based on what TID userspace gave us. * We dont trust it. */ static struct task_struct * futex_find_get_task(pid_t pid) { struct task_struct *p; rcu_read_lock(); p = find_task_by_vpid(pid); if (p) get_task_struct(p); rcu_read_unlock(); return p; } /* * This task is holding PI mutexes at exit time => bad. * Kernel cleans up PI-state, but userspace is likely hosed. * (Robust-futex cleanup is separate and might save the day for userspace.) */ void exit_pi_state_list(struct task_struct *curr) { struct list_head *next, *head = &curr->pi_state_list; struct futex_pi_state *pi_state; struct futex_hash_bucket *hb; union futex_key key = FUTEX_KEY_INIT; if (!futex_cmpxchg_enabled) return; /* * We are a ZOMBIE and nobody can enqueue itself on * pi_state_list anymore, but we have to be careful * versus waiters unqueueing themselves: */ raw_spin_lock_irq(&curr->pi_lock); while (!list_empty(head)) { next = head->next; pi_state = list_entry(next, struct futex_pi_state, list); key = pi_state->key; hb = hash_futex(&key); raw_spin_unlock_irq(&curr->pi_lock); spin_lock(&hb->lock); raw_spin_lock_irq(&curr->pi_lock); /* * We dropped the pi-lock, so re-check whether this * task still owns the PI-state: */ if (head->next != next) { spin_unlock(&hb->lock); continue; } WARN_ON(pi_state->owner != curr); WARN_ON(list_empty(&pi_state->list)); list_del_init(&pi_state->list); pi_state->owner = NULL; raw_spin_unlock_irq(&curr->pi_lock); rt_mutex_unlock(&pi_state->pi_mutex); spin_unlock(&hb->lock); raw_spin_lock_irq(&curr->pi_lock); } raw_spin_unlock_irq(&curr->pi_lock); } static int lookup_pi_state(u32 uval, struct futex_hash_bucket *hb, union futex_key *key, struct futex_pi_state **ps) { struct futex_pi_state *pi_state = NULL; struct futex_q *this, *next; struct plist_head *head; struct task_struct *p; pid_t pid = uval & FUTEX_TID_MASK; head = &hb->chain; plist_for_each_entry_safe(this, next, head, list) { if (match_futex(&this->key, key)) { /* * Another waiter already exists - bump up * the refcount and return its pi_state: */ pi_state = this->pi_state; /* * Userspace might have messed up non-PI and PI futexes */ if (unlikely(!pi_state)) return -EINVAL; WARN_ON(!atomic_read(&pi_state->refcount)); /* * When pi_state->owner is NULL then the owner died * and another waiter is on the fly. pi_state->owner * is fixed up by the task which acquires * pi_state->rt_mutex. * * We do not check for pid == 0 which can happen when * the owner died and robust_list_exit() cleared the * TID. */ if (pid && pi_state->owner) { /* * Bail out if user space manipulated the * futex value. */ if (pid != task_pid_vnr(pi_state->owner)) return -EINVAL; } atomic_inc(&pi_state->refcount); *ps = pi_state; return 0; } } /* * We are the first waiter - try to look up the real owner and attach * the new pi_state to it, but bail out when TID = 0 */ if (!pid) return -ESRCH; p = futex_find_get_task(pid); if (!p) return -ESRCH; /* * We need to look at the task state flags to figure out, * whether the task is exiting. To protect against the do_exit * change of the task flags, we do this protected by * p->pi_lock: */ raw_spin_lock_irq(&p->pi_lock); if (unlikely(p->flags & PF_EXITING)) { /* * The task is on the way out. When PF_EXITPIDONE is * set, we know that the task has finished the * cleanup: */ int ret = (p->flags & PF_EXITPIDONE) ? -ESRCH : -EAGAIN; raw_spin_unlock_irq(&p->pi_lock); put_task_struct(p); return ret; } pi_state = alloc_pi_state(); /* * Initialize the pi_mutex in locked state and make 'p' * the owner of it: */ rt_mutex_init_proxy_locked(&pi_state->pi_mutex, p); /* Store the key for possible exit cleanups: */ pi_state->key = *key; WARN_ON(!list_empty(&pi_state->list)); list_add(&pi_state->list, &p->pi_state_list); pi_state->owner = p; raw_spin_unlock_irq(&p->pi_lock); put_task_struct(p); *ps = pi_state; return 0; } /** * futex_lock_pi_atomic() - Atomic work required to acquire a pi aware futex * @uaddr: the pi futex user address * @hb: the pi futex hash bucket * @key: the futex key associated with uaddr and hb * @ps: the pi_state pointer where we store the result of the * lookup * @task: the task to perform the atomic lock work for. This will * be "current" except in the case of requeue pi. * @set_waiters: force setting the FUTEX_WAITERS bit (1) or not (0) * * Returns: * 0 - ready to wait * 1 - acquired the lock * <0 - error * * The hb->lock and futex_key refs shall be held by the caller. */ static int futex_lock_pi_atomic(u32 __user *uaddr, struct futex_hash_bucket *hb, union futex_key *key, struct futex_pi_state **ps, struct task_struct *task, int set_waiters) { int lock_taken, ret, force_take = 0; u32 uval, newval, curval, vpid = task_pid_vnr(task); retry: ret = lock_taken = 0; /* * To avoid races, we attempt to take the lock here again * (by doing a 0 -> TID atomic cmpxchg), while holding all * the locks. It will most likely not succeed. */ newval = vpid; if (set_waiters) newval |= FUTEX_WAITERS; if (unlikely(cmpxchg_futex_value_locked(&curval, uaddr, 0, newval))) return -EFAULT; /* * Detect deadlocks. */ if ((unlikely((curval & FUTEX_TID_MASK) == vpid))) return -EDEADLK; /* * Surprise - we got the lock. Just return to userspace: */ if (unlikely(!curval)) return 1; uval = curval; /* * Set the FUTEX_WAITERS flag, so the owner will know it has someone * to wake at the next unlock. */ newval = curval | FUTEX_WAITERS; /* * Should we force take the futex? See below. */ if (unlikely(force_take)) { /* * Keep the OWNER_DIED and the WAITERS bit and set the * new TID value. */ newval = (curval & ~FUTEX_TID_MASK) | vpid; force_take = 0; lock_taken = 1; } if (unlikely(cmpxchg_futex_value_locked(&curval, uaddr, uval, newval))) return -EFAULT; if (unlikely(curval != uval)) goto retry; /* * We took the lock due to forced take over. */ if (unlikely(lock_taken)) return 1; /* * We dont have the lock. Look up the PI state (or create it if * we are the first waiter): */ ret = lookup_pi_state(uval, hb, key, ps); if (unlikely(ret)) { switch (ret) { case -ESRCH: /* * We failed to find an owner for this * futex. So we have no pi_state to block * on. This can happen in two cases: * * 1) The owner died * 2) A stale FUTEX_WAITERS bit * * Re-read the futex value. */ if (get_futex_value_locked(&curval, uaddr)) return -EFAULT; /* * If the owner died or we have a stale * WAITERS bit the owner TID in the user space * futex is 0. */ if (!(curval & FUTEX_TID_MASK)) { force_take = 1; goto retry; } default: break; } } return ret; } /** * __unqueue_futex() - Remove the futex_q from its futex_hash_bucket * @q: The futex_q to unqueue * * The q->lock_ptr must not be NULL and must be held by the caller. */ static void __unqueue_futex(struct futex_q *q) { struct futex_hash_bucket *hb; if (WARN_ON_SMP(!q->lock_ptr || !spin_is_locked(q->lock_ptr)) || WARN_ON(plist_node_empty(&q->list))) return; hb = container_of(q->lock_ptr, struct futex_hash_bucket, lock); plist_del(&q->list, &hb->chain); } /* * The hash bucket lock must be held when this is called. * Afterwards, the futex_q must not be accessed. */ static void wake_futex(struct futex_q *q) { struct task_struct *p = q->task; if (WARN(q->pi_state || q->rt_waiter, "refusing to wake PI futex\n")) return; /* * We set q->lock_ptr = NULL _before_ we wake up the task. If * a non-futex wake up happens on another CPU then the task * might exit and p would dereference a non-existing task * struct. Prevent this by holding a reference on p across the * wake up. */ get_task_struct(p); __unqueue_futex(q); /* * The waiting task can free the futex_q as soon as * q->lock_ptr = NULL is written, without taking any locks. A * memory barrier is required here to prevent the following * store to lock_ptr from getting ahead of the plist_del. */ smp_wmb(); q->lock_ptr = NULL; wake_up_state(p, TASK_NORMAL); put_task_struct(p); } static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *this) { struct task_struct *new_owner; struct futex_pi_state *pi_state = this->pi_state; u32 uninitialized_var(curval), newval; if (!pi_state) return -EINVAL; /* * If current does not own the pi_state then the futex is * inconsistent and user space fiddled with the futex value. */ if (pi_state->owner != current) return -EINVAL; raw_spin_lock(&pi_state->pi_mutex.wait_lock); new_owner = rt_mutex_next_owner(&pi_state->pi_mutex); /* * It is possible that the next waiter (the one that brought * this owner to the kernel) timed out and is no longer * waiting on the lock. */ if (!new_owner) new_owner = this->task; /* * We pass it to the next owner. (The WAITERS bit is always * kept enabled while there is PI state around. We must also * preserve the owner died bit.) */ if (!(uval & FUTEX_OWNER_DIED)) { int ret = 0; newval = FUTEX_WAITERS | task_pid_vnr(new_owner); if (cmpxchg_futex_value_locked(&curval, uaddr, uval, newval)) ret = -EFAULT; else if (curval != uval) ret = -EINVAL; if (ret) { raw_spin_unlock(&pi_state->pi_mutex.wait_lock); return ret; } } raw_spin_lock_irq(&pi_state->owner->pi_lock); WARN_ON(list_empty(&pi_state->list)); list_del_init(&pi_state->list); raw_spin_unlock_irq(&pi_state->owner->pi_lock); raw_spin_lock_irq(&new_owner->pi_lock); WARN_ON(!list_empty(&pi_state->list)); list_add(&pi_state->list, &new_owner->pi_state_list); pi_state->owner = new_owner; raw_spin_unlock_irq(&new_owner->pi_lock); raw_spin_unlock(&pi_state->pi_mutex.wait_lock); rt_mutex_unlock(&pi_state->pi_mutex); return 0; } static int unlock_futex_pi(u32 __user *uaddr, u32 uval) { u32 uninitialized_var(oldval); /* * There is no waiter, so we unlock the futex. The owner died * bit has not to be preserved here. We are the owner: */ if (cmpxchg_futex_value_locked(&oldval, uaddr, uval, 0)) return -EFAULT; if (oldval != uval) return -EAGAIN; return 0; } /* * Express the locking dependencies for lockdep: */ static inline void double_lock_hb(struct futex_hash_bucket *hb1, struct futex_hash_bucket *hb2) { if (hb1 <= hb2) { spin_lock(&hb1->lock); if (hb1 < hb2) spin_lock_nested(&hb2->lock, SINGLE_DEPTH_NESTING); } else { /* hb1 > hb2 */ spin_lock(&hb2->lock); spin_lock_nested(&hb1->lock, SINGLE_DEPTH_NESTING); } } static inline void double_unlock_hb(struct futex_hash_bucket *hb1, struct futex_hash_bucket *hb2) { spin_unlock(&hb1->lock); if (hb1 != hb2) spin_unlock(&hb2->lock); } /* * Wake up waiters matching bitset queued on this futex (uaddr). */ static int futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset) { struct futex_hash_bucket *hb; struct futex_q *this, *next; struct plist_head *head; union futex_key key = FUTEX_KEY_INIT; int ret; if (!bitset) return -EINVAL; ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &key, VERIFY_READ); if (unlikely(ret != 0)) goto out; hb = hash_futex(&key); spin_lock(&hb->lock); head = &hb->chain; plist_for_each_entry_safe(this, next, head, list) { if (match_futex (&this->key, &key)) { if (this->pi_state || this->rt_waiter) { ret = -EINVAL; break; } /* Check if one of the bits is set in both bitsets */ if (!(this->bitset & bitset)) continue; wake_futex(this); if (++ret >= nr_wake) break; } } spin_unlock(&hb->lock); put_futex_key(&key); out: return ret; } /* * Wake up all waiters hashed on the physical page that is mapped * to this virtual address: */ static int futex_wake_op(u32 __user *uaddr1, unsigned int flags, u32 __user *uaddr2, int nr_wake, int nr_wake2, int op) { union futex_key key1 = FUTEX_KEY_INIT, key2 = FUTEX_KEY_INIT; struct futex_hash_bucket *hb1, *hb2; struct plist_head *head; struct futex_q *this, *next; int ret, op_ret; retry: ret = get_futex_key(uaddr1, flags & FLAGS_SHARED, &key1, VERIFY_READ); if (unlikely(ret != 0)) goto out; ret = get_futex_key(uaddr2, flags & FLAGS_SHARED, &key2, VERIFY_WRITE); if (unlikely(ret != 0)) goto out_put_key1; hb1 = hash_futex(&key1); hb2 = hash_futex(&key2); retry_private: double_lock_hb(hb1, hb2); op_ret = futex_atomic_op_inuser(op, uaddr2); if (unlikely(op_ret < 0)) { double_unlock_hb(hb1, hb2); #ifndef CONFIG_MMU /* * we don't get EFAULT from MMU faults if we don't have an MMU, * but we might get them from range checking */ ret = op_ret; goto out_put_keys; #endif if (unlikely(op_ret != -EFAULT)) { ret = op_ret; goto out_put_keys; } ret = fault_in_user_writeable(uaddr2); if (ret) goto out_put_keys; if (!(flags & FLAGS_SHARED)) goto retry_private; put_futex_key(&key2); put_futex_key(&key1); goto retry; } head = &hb1->chain; plist_for_each_entry_safe(this, next, head, list) { if (match_futex (&this->key, &key1)) { if (this->pi_state || this->rt_waiter) { ret = -EINVAL; goto out_unlock; } wake_futex(this); if (++ret >= nr_wake) break; } } if (op_ret > 0) { head = &hb2->chain; op_ret = 0; plist_for_each_entry_safe(this, next, head, list) { if (match_futex (&this->key, &key2)) { if (this->pi_state || this->rt_waiter) { ret = -EINVAL; goto out_unlock; } wake_futex(this); if (++op_ret >= nr_wake2) break; } } ret += op_ret; } out_unlock: double_unlock_hb(hb1, hb2); out_put_keys: put_futex_key(&key2); out_put_key1: put_futex_key(&key1); out: return ret; } /** * requeue_futex() - Requeue a futex_q from one hb to another * @q: the futex_q to requeue * @hb1: the source hash_bucket * @hb2: the target hash_bucket * @key2: the new key for the requeued futex_q */ static inline void requeue_futex(struct futex_q *q, struct futex_hash_bucket *hb1, struct futex_hash_bucket *hb2, union futex_key *key2) { /* * If key1 and key2 hash to the same bucket, no need to * requeue. */ if (likely(&hb1->chain != &hb2->chain)) { plist_del(&q->list, &hb1->chain); plist_add(&q->list, &hb2->chain); q->lock_ptr = &hb2->lock; } get_futex_key_refs(key2); q->key = *key2; } /** * requeue_pi_wake_futex() - Wake a task that acquired the lock during requeue * @q: the futex_q * @key: the key of the requeue target futex * @hb: the hash_bucket of the requeue target futex * * During futex_requeue, with requeue_pi=1, it is possible to acquire the * target futex if it is uncontended or via a lock steal. Set the futex_q key * to the requeue target futex so the waiter can detect the wakeup on the right * futex, but remove it from the hb and NULL the rt_waiter so it can detect * atomic lock acquisition. Set the q->lock_ptr to the requeue target hb->lock * to protect access to the pi_state to fixup the owner later. Must be called * with both q->lock_ptr and hb->lock held. */ static inline void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key, struct futex_hash_bucket *hb) { get_futex_key_refs(key); q->key = *key; __unqueue_futex(q); WARN_ON(!q->rt_waiter); q->rt_waiter = NULL; q->lock_ptr = &hb->lock; wake_up_state(q->task, TASK_NORMAL); } /** * futex_proxy_trylock_atomic() - Attempt an atomic lock for the top waiter * @pifutex: the user address of the to futex * @hb1: the from futex hash bucket, must be locked by the caller * @hb2: the to futex hash bucket, must be locked by the caller * @key1: the from futex key * @key2: the to futex key * @ps: address to store the pi_state pointer * @set_waiters: force setting the FUTEX_WAITERS bit (1) or not (0) * * Try and get the lock on behalf of the top waiter if we can do it atomically. * Wake the top waiter if we succeed. If the caller specified set_waiters, * then direct futex_lock_pi_atomic() to force setting the FUTEX_WAITERS bit. * hb1 and hb2 must be held by the caller. * * Returns: * 0 - failed to acquire the lock atomicly * 1 - acquired the lock * <0 - error */ static int futex_proxy_trylock_atomic(u32 __user *pifutex, struct futex_hash_bucket *hb1, struct futex_hash_bucket *hb2, union futex_key *key1, union futex_key *key2, struct futex_pi_state **ps, int set_waiters) { struct futex_q *top_waiter = NULL; u32 curval; int ret; if (get_futex_value_locked(&curval, pifutex)) return -EFAULT; /* * Find the top_waiter and determine if there are additional waiters. * If the caller intends to requeue more than 1 waiter to pifutex, * force futex_lock_pi_atomic() to set the FUTEX_WAITERS bit now, * as we have means to handle the possible fault. If not, don't set * the bit unecessarily as it will force the subsequent unlock to enter * the kernel. */ top_waiter = futex_top_waiter(hb1, key1); /* There are no waiters, nothing for us to do. */ if (!top_waiter) return 0; /* Ensure we requeue to the expected futex. */ if (!match_futex(top_waiter->requeue_pi_key, key2)) return -EINVAL; /* * Try to take the lock for top_waiter. Set the FUTEX_WAITERS bit in * the contended case or if set_waiters is 1. The pi_state is returned * in ps in contended cases. */ ret = futex_lock_pi_atomic(pifutex, hb2, key2, ps, top_waiter->task, set_waiters); if (ret == 1) requeue_pi_wake_futex(top_waiter, key2, hb2); return ret; } /** * futex_requeue() - Requeue waiters from uaddr1 to uaddr2 * @uaddr1: source futex user address * @flags: futex flags (FLAGS_SHARED, etc.) * @uaddr2: target futex user address * @nr_wake: number of waiters to wake (must be 1 for requeue_pi) * @nr_requeue: number of waiters to requeue (0-INT_MAX) * @cmpval: @uaddr1 expected value (or %NULL) * @requeue_pi: if we are attempting to requeue from a non-pi futex to a * pi futex (pi to pi requeue is not supported) * * Requeue waiters on uaddr1 to uaddr2. In the requeue_pi case, try to acquire * uaddr2 atomically on behalf of the top waiter. * * Returns: * >=0 - on success, the number of tasks requeued or woken * <0 - on error */ static int futex_requeue(u32 __user *uaddr1, unsigned int flags, u32 __user *uaddr2, int nr_wake, int nr_requeue, u32 *cmpval, int requeue_pi) { union futex_key key1 = FUTEX_KEY_INIT, key2 = FUTEX_KEY_INIT; int drop_count = 0, task_count = 0, ret; struct futex_pi_state *pi_state = NULL; struct futex_hash_bucket *hb1, *hb2; struct plist_head *head1; struct futex_q *this, *next; u32 curval2; if (requeue_pi) { /* * requeue_pi requires a pi_state, try to allocate it now * without any locks in case it fails. */ if (refill_pi_state_cache()) return -ENOMEM; /* * requeue_pi must wake as many tasks as it can, up to nr_wake * + nr_requeue, since it acquires the rt_mutex prior to * returning to userspace, so as to not leave the rt_mutex with * waiters and no owner. However, second and third wake-ups * cannot be predicted as they involve race conditions with the * first wake and a fault while looking up the pi_state. Both * pthread_cond_signal() and pthread_cond_broadcast() should * use nr_wake=1. */ if (nr_wake != 1) return -EINVAL; } retry: if (pi_state != NULL) { /* * We will have to lookup the pi_state again, so free this one * to keep the accounting correct. */ free_pi_state(pi_state); pi_state = NULL; } ret = get_futex_key(uaddr1, flags & FLAGS_SHARED, &key1, VERIFY_READ); if (unlikely(ret != 0)) goto out; ret = get_futex_key(uaddr2, flags & FLAGS_SHARED, &key2, requeue_pi ? VERIFY_WRITE : VERIFY_READ); if (unlikely(ret != 0)) goto out_put_key1; hb1 = hash_futex(&key1); hb2 = hash_futex(&key2); retry_private: double_lock_hb(hb1, hb2); if (likely(cmpval != NULL)) { u32 curval; ret = get_futex_value_locked(&curval, uaddr1); if (unlikely(ret)) { double_unlock_hb(hb1, hb2); ret = get_user(curval, uaddr1); if (ret) goto out_put_keys; if (!(flags & FLAGS_SHARED)) goto retry_private; put_futex_key(&key2); put_futex_key(&key1); goto retry; } if (curval != *cmpval) { ret = -EAGAIN; goto out_unlock; } } if (requeue_pi && (task_count - nr_wake < nr_requeue)) { /* * Attempt to acquire uaddr2 and wake the top waiter. If we * intend to requeue waiters, force setting the FUTEX_WAITERS * bit. We force this here where we are able to easily handle * faults rather in the requeue loop below. */ ret = futex_proxy_trylock_atomic(uaddr2, hb1, hb2, &key1, &key2, &pi_state, nr_requeue); /* * At this point the top_waiter has either taken uaddr2 or is * waiting on it. If the former, then the pi_state will not * exist yet, look it up one more time to ensure we have a * reference to it. */ if (ret == 1) { WARN_ON(pi_state); drop_count++; task_count++; ret = get_futex_value_locked(&curval2, uaddr2); if (!ret) ret = lookup_pi_state(curval2, hb2, &key2, &pi_state); } switch (ret) { case 0: break; case -EFAULT: double_unlock_hb(hb1, hb2); put_futex_key(&key2); put_futex_key(&key1); ret = fault_in_user_writeable(uaddr2); if (!ret) goto retry; goto out; case -EAGAIN: /* The owner was exiting, try again. */ double_unlock_hb(hb1, hb2); put_futex_key(&key2); put_futex_key(&key1); cond_resched(); goto retry; default: goto out_unlock; } } head1 = &hb1->chain; plist_for_each_entry_safe(this, next, head1, list) { if (task_count - nr_wake >= nr_requeue) break; if (!match_futex(&this->key, &key1)) continue; /* * FUTEX_WAIT_REQEUE_PI and FUTEX_CMP_REQUEUE_PI should always * be paired with each other and no other futex ops. * * We should never be requeueing a futex_q with a pi_state, * which is awaiting a futex_unlock_pi(). */ if ((requeue_pi && !this->rt_waiter) || (!requeue_pi && this->rt_waiter) || this->pi_state) { ret = -EINVAL; break; } /* * Wake nr_wake waiters. For requeue_pi, if we acquired the * lock, we already woke the top_waiter. If not, it will be * woken by futex_unlock_pi(). */ if (++task_count <= nr_wake && !requeue_pi) { wake_futex(this); continue; } /* Ensure we requeue to the expected futex for requeue_pi. */ if (requeue_pi && !match_futex(this->requeue_pi_key, &key2)) { ret = -EINVAL; break; } /* * Requeue nr_requeue waiters and possibly one more in the case * of requeue_pi if we couldn't acquire the lock atomically. */ if (requeue_pi) { /* Prepare the waiter to take the rt_mutex. */ atomic_inc(&pi_state->refcount); this->pi_state = pi_state; ret = rt_mutex_start_proxy_lock(&pi_state->pi_mutex, this->rt_waiter, this->task, 1); if (ret == 1) { /* We got the lock. */ requeue_pi_wake_futex(this, &key2, hb2); drop_count++; continue; } else if (ret) { /* -EDEADLK */ this->pi_state = NULL; free_pi_state(pi_state); goto out_unlock; } } requeue_futex(this, hb1, hb2, &key2); drop_count++; } out_unlock: double_unlock_hb(hb1, hb2); /* * drop_futex_key_refs() must be called outside the spinlocks. During * the requeue we moved futex_q's from the hash bucket at key1 to the * one at key2 and updated their key pointer. We no longer need to * hold the references to key1. */ while (--drop_count >= 0) drop_futex_key_refs(&key1); out_put_keys: put_futex_key(&key2); out_put_key1: put_futex_key(&key1); out: if (pi_state != NULL) free_pi_state(pi_state); return ret ? ret : task_count; } /* The key must be already stored in q->key. */ static inline struct futex_hash_bucket *queue_lock(struct futex_q *q) __acquires(&hb->lock) { struct futex_hash_bucket *hb; hb = hash_futex(&q->key); q->lock_ptr = &hb->lock; spin_lock(&hb->lock); return hb; } static inline void queue_unlock(struct futex_q *q, struct futex_hash_bucket *hb) __releases(&hb->lock) { spin_unlock(&hb->lock); } /** * queue_me() - Enqueue the futex_q on the futex_hash_bucket * @q: The futex_q to enqueue * @hb: The destination hash bucket * * The hb->lock must be held by the caller, and is released here. A call to * queue_me() is typically paired with exactly one call to unqueue_me(). The * exceptions involve the PI related operations, which may use unqueue_me_pi() * or nothing if the unqueue is done as part of the wake process and the unqueue * state is implicit in the state of woken task (see futex_wait_requeue_pi() for * an example). */ static inline void queue_me(struct futex_q *q, struct futex_hash_bucket *hb) __releases(&hb->lock) { int prio; /* * The priority used to register this element is * - either the real thread-priority for the real-time threads * (i.e. threads with a priority lower than MAX_RT_PRIO) * - or MAX_RT_PRIO for non-RT threads. * Thus, all RT-threads are woken first in priority order, and * the others are woken last, in FIFO order. */ prio = min(current->normal_prio, MAX_RT_PRIO); plist_node_init(&q->list, prio); plist_add(&q->list, &hb->chain); q->task = current; spin_unlock(&hb->lock); } /** * unqueue_me() - Remove the futex_q from its futex_hash_bucket * @q: The futex_q to unqueue * * The q->lock_ptr must not be held by the caller. A call to unqueue_me() must * be paired with exactly one earlier call to queue_me(). * * Returns: * 1 - if the futex_q was still queued (and we removed unqueued it) * 0 - if the futex_q was already removed by the waking thread */ static int unqueue_me(struct futex_q *q) { spinlock_t *lock_ptr; int ret = 0; /* In the common case we don't take the spinlock, which is nice. */ retry: lock_ptr = q->lock_ptr; barrier(); if (lock_ptr != NULL) { spin_lock(lock_ptr); /* * q->lock_ptr can change between reading it and * spin_lock(), causing us to take the wrong lock. This * corrects the race condition. * * Reasoning goes like this: if we have the wrong lock, * q->lock_ptr must have changed (maybe several times) * between reading it and the spin_lock(). It can * change again after the spin_lock() but only if it was * already changed before the spin_lock(). It cannot, * however, change back to the original value. Therefore * we can detect whether we acquired the correct lock. */ if (unlikely(lock_ptr != q->lock_ptr)) { spin_unlock(lock_ptr); goto retry; } __unqueue_futex(q); BUG_ON(q->pi_state); spin_unlock(lock_ptr); ret = 1; } drop_futex_key_refs(&q->key); return ret; } /* * PI futexes can not be requeued and must remove themself from the * hash bucket. The hash bucket lock (i.e. lock_ptr) is held on entry * and dropped here. */ static void unqueue_me_pi(struct futex_q *q) __releases(q->lock_ptr) { __unqueue_futex(q); BUG_ON(!q->pi_state); free_pi_state(q->pi_state); q->pi_state = NULL; spin_unlock(q->lock_ptr); } /* * Fixup the pi_state owner with the new owner. * * Must be called with hash bucket lock held and mm->sem held for non * private futexes. */ static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q, struct task_struct *newowner) { u32 newtid = task_pid_vnr(newowner) | FUTEX_WAITERS; struct futex_pi_state *pi_state = q->pi_state; struct task_struct *oldowner = pi_state->owner; u32 uval, uninitialized_var(curval), newval; int ret; /* Owner died? */ if (!pi_state->owner) newtid |= FUTEX_OWNER_DIED; /* * We are here either because we stole the rtmutex from the * previous highest priority waiter or we are the highest priority * waiter but failed to get the rtmutex the first time. * We have to replace the newowner TID in the user space variable. * This must be atomic as we have to preserve the owner died bit here. * * Note: We write the user space value _before_ changing the pi_state * because we can fault here. Imagine swapped out pages or a fork * that marked all the anonymous memory readonly for cow. * * Modifying pi_state _before_ the user space value would * leave the pi_state in an inconsistent state when we fault * here, because we need to drop the hash bucket lock to * handle the fault. This might be observed in the PID check * in lookup_pi_state. */ retry: if (get_futex_value_locked(&uval, uaddr)) goto handle_fault; while (1) { newval = (uval & FUTEX_OWNER_DIED) | newtid; if (cmpxchg_futex_value_locked(&curval, uaddr, uval, newval)) goto handle_fault; if (curval == uval) break; uval = curval; } /* * We fixed up user space. Now we need to fix the pi_state * itself. */ if (pi_state->owner != NULL) { raw_spin_lock_irq(&pi_state->owner->pi_lock); WARN_ON(list_empty(&pi_state->list)); list_del_init(&pi_state->list); raw_spin_unlock_irq(&pi_state->owner->pi_lock); } pi_state->owner = newowner; raw_spin_lock_irq(&newowner->pi_lock); WARN_ON(!list_empty(&pi_state->list)); list_add(&pi_state->list, &newowner->pi_state_list); raw_spin_unlock_irq(&newowner->pi_lock); return 0; /* * To handle the page fault we need to drop the hash bucket * lock here. That gives the other task (either the highest priority * waiter itself or the task which stole the rtmutex) the * chance to try the fixup of the pi_state. So once we are * back from handling the fault we need to check the pi_state * after reacquiring the hash bucket lock and before trying to * do another fixup. When the fixup has been done already we * simply return. */ handle_fault: spin_unlock(q->lock_ptr); ret = fault_in_user_writeable(uaddr); spin_lock(q->lock_ptr); /* * Check if someone else fixed it for us: */ if (pi_state->owner != oldowner) return 0; if (ret) return ret; goto retry; } static long futex_wait_restart(struct restart_block *restart); /** * fixup_owner() - Post lock pi_state and corner case management * @uaddr: user address of the futex * @q: futex_q (contains pi_state and access to the rt_mutex) * @locked: if the attempt to take the rt_mutex succeeded (1) or not (0) * * After attempting to lock an rt_mutex, this function is called to cleanup * the pi_state owner as well as handle race conditions that may allow us to * acquire the lock. Must be called with the hb lock held. * * Returns: * 1 - success, lock taken * 0 - success, lock not taken * <0 - on error (-EFAULT) */ static int fixup_owner(u32 __user *uaddr, struct futex_q *q, int locked) { struct task_struct *owner; int ret = 0; if (locked) { /* * Got the lock. We might not be the anticipated owner if we * did a lock-steal - fix up the PI-state in that case: */ if (q->pi_state->owner != current) ret = fixup_pi_state_owner(uaddr, q, current); goto out; } /* * Catch the rare case, where the lock was released when we were on the * way back before we locked the hash bucket. */ if (q->pi_state->owner == current) { /* * Try to get the rt_mutex now. This might fail as some other * task acquired the rt_mutex after we removed ourself from the * rt_mutex waiters list. */ if (rt_mutex_trylock(&q->pi_state->pi_mutex)) { locked = 1; goto out; } /* * pi_state is incorrect, some other task did a lock steal and * we returned due to timeout or signal without taking the * rt_mutex. Too late. */ raw_spin_lock(&q->pi_state->pi_mutex.wait_lock); owner = rt_mutex_owner(&q->pi_state->pi_mutex); if (!owner) owner = rt_mutex_next_owner(&q->pi_state->pi_mutex); raw_spin_unlock(&q->pi_state->pi_mutex.wait_lock); ret = fixup_pi_state_owner(uaddr, q, owner); goto out; } /* * Paranoia check. If we did not take the lock, then we should not be * the owner of the rt_mutex. */ if (rt_mutex_owner(&q->pi_state->pi_mutex) == current) printk(KERN_ERR "fixup_owner: ret = %d pi-mutex: %p " "pi-state %p\n", ret, q->pi_state->pi_mutex.owner, q->pi_state->owner); out: return ret ? ret : locked; } /** * futex_wait_queue_me() - queue_me() and wait for wakeup, timeout, or signal * @hb: the futex hash bucket, must be locked by the caller * @q: the futex_q to queue up on * @timeout: the prepared hrtimer_sleeper, or null for no timeout */ static void futex_wait_queue_me(struct futex_hash_bucket *hb, struct futex_q *q, struct hrtimer_sleeper *timeout) { /* * The task state is guaranteed to be set before another task can * wake it. set_current_state() is implemented using set_mb() and * queue_me() calls spin_unlock() upon completion, both serializing * access to the hash list and forcing another memory barrier. */ set_current_state(TASK_INTERRUPTIBLE); queue_me(q, hb); /* Arm the timer */ if (timeout) { hrtimer_start_expires(&timeout->timer, HRTIMER_MODE_ABS); if (!hrtimer_active(&timeout->timer)) timeout->task = NULL; } /* * If we have been removed from the hash list, then another task * has tried to wake us, and we can skip the call to schedule(). */ if (likely(!plist_node_empty(&q->list))) { /* * If the timer has already expired, current will already be * flagged for rescheduling. Only call schedule if there * is no timeout, or if it has yet to expire. */ if (!timeout || timeout->task) schedule(); } __set_current_state(TASK_RUNNING); } /** * futex_wait_setup() - Prepare to wait on a futex * @uaddr: the futex userspace address * @val: the expected value * @flags: futex flags (FLAGS_SHARED, etc.) * @q: the associated futex_q * @hb: storage for hash_bucket pointer to be returned to caller * * Setup the futex_q and locate the hash_bucket. Get the futex value and * compare it with the expected value. Handle atomic faults internally. * Return with the hb lock held and a q.key reference on success, and unlocked * with no q.key reference on failure. * * Returns: * 0 - uaddr contains val and hb has been locked * <1 - -EFAULT or -EWOULDBLOCK (uaddr does not contain val) and hb is unlocked */ static int futex_wait_setup(u32 __user *uaddr, u32 val, unsigned int flags, struct futex_q *q, struct futex_hash_bucket **hb) { u32 uval; int ret; /* * Access the page AFTER the hash-bucket is locked. * Order is important: * * Userspace waiter: val = var; if (cond(val)) futex_wait(&var, val); * Userspace waker: if (cond(var)) { var = new; futex_wake(&var); } * * The basic logical guarantee of a futex is that it blocks ONLY * if cond(var) is known to be true at the time of blocking, for * any cond. If we locked the hash-bucket after testing *uaddr, that * would open a race condition where we could block indefinitely with * cond(var) false, which would violate the guarantee. * * On the other hand, we insert q and release the hash-bucket only * after testing *uaddr. This guarantees that futex_wait() will NOT * absorb a wakeup if *uaddr does not match the desired values * while the syscall executes. */ retry: ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &q->key, VERIFY_READ); if (unlikely(ret != 0)) return ret; retry_private: *hb = queue_lock(q); ret = get_futex_value_locked(&uval, uaddr); if (ret) { queue_unlock(q, *hb); ret = get_user(uval, uaddr); if (ret) goto out; if (!(flags & FLAGS_SHARED)) goto retry_private; put_futex_key(&q->key); goto retry; } if (uval != val) { queue_unlock(q, *hb); ret = -EWOULDBLOCK; } out: if (ret) put_futex_key(&q->key); return ret; } static int futex_wait(u32 __user *uaddr, unsigned int flags, u32 val, ktime_t *abs_time, u32 bitset) { struct hrtimer_sleeper timeout, *to = NULL; struct restart_block *restart; struct futex_hash_bucket *hb; struct futex_q q = futex_q_init; int ret; if (!bitset) return -EINVAL; q.bitset = bitset; if (abs_time) { to = &timeout; hrtimer_init_on_stack(&to->timer, (flags & FLAGS_CLOCKRT) ? CLOCK_REALTIME : CLOCK_MONOTONIC, HRTIMER_MODE_ABS); hrtimer_init_sleeper(to, current); hrtimer_set_expires_range_ns(&to->timer, *abs_time, current->timer_slack_ns); } retry: /* * Prepare to wait on uaddr. On success, holds hb lock and increments * q.key refs. */ ret = futex_wait_setup(uaddr, val, flags, &q, &hb); if (ret) goto out; /* queue_me and wait for wakeup, timeout, or a signal. */ futex_wait_queue_me(hb, &q, to); /* If we were woken (and unqueued), we succeeded, whatever. */ ret = 0; /* unqueue_me() drops q.key ref */ if (!unqueue_me(&q)) goto out; ret = -ETIMEDOUT; if (to && !to->task) goto out; /* * We expect signal_pending(current), but we might be the * victim of a spurious wakeup as well. */ if (!signal_pending(current)) goto retry; ret = -ERESTARTSYS; if (!abs_time) goto out; restart = ¤t_thread_info()->restart_block; restart->fn = futex_wait_restart; restart->futex.uaddr = uaddr; restart->futex.val = val; restart->futex.time = abs_time->tv64; restart->futex.bitset = bitset; restart->futex.flags = flags | FLAGS_HAS_TIMEOUT; ret = -ERESTART_RESTARTBLOCK; out: if (to) { hrtimer_cancel(&to->timer); destroy_hrtimer_on_stack(&to->timer); } return ret; } static long futex_wait_restart(struct restart_block *restart) { u32 __user *uaddr = restart->futex.uaddr; ktime_t t, *tp = NULL; if (restart->futex.flags & FLAGS_HAS_TIMEOUT) { t.tv64 = restart->futex.time; tp = &t; } restart->fn = do_no_restart_syscall; return (long)futex_wait(uaddr, restart->futex.flags, restart->futex.val, tp, restart->futex.bitset); } /* * Userspace tried a 0 -> TID atomic transition of the futex value * and failed. The kernel side here does the whole locking operation: * if there are waiters then it will block, it does PI, etc. (Due to * races the kernel might see a 0 value of the futex too.) */ static int futex_lock_pi(u32 __user *uaddr, unsigned int flags, int detect, ktime_t *time, int trylock) { struct hrtimer_sleeper timeout, *to = NULL; struct futex_hash_bucket *hb; struct futex_q q = futex_q_init; int res, ret; if (refill_pi_state_cache()) return -ENOMEM; if (time) { to = &timeout; hrtimer_init_on_stack(&to->timer, CLOCK_REALTIME, HRTIMER_MODE_ABS); hrtimer_init_sleeper(to, current); hrtimer_set_expires(&to->timer, *time); } retry: ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &q.key, VERIFY_WRITE); if (unlikely(ret != 0)) goto out; retry_private: hb = queue_lock(&q); ret = futex_lock_pi_atomic(uaddr, hb, &q.key, &q.pi_state, current, 0); if (unlikely(ret)) { switch (ret) { case 1: /* We got the lock. */ ret = 0; goto out_unlock_put_key; case -EFAULT: goto uaddr_faulted; case -EAGAIN: /* * Task is exiting and we just wait for the * exit to complete. */ queue_unlock(&q, hb); put_futex_key(&q.key); cond_resched(); goto retry; default: goto out_unlock_put_key; } } /* * Only actually queue now that the atomic ops are done: */ queue_me(&q, hb); WARN_ON(!q.pi_state); /* * Block on the PI mutex: */ if (!trylock) ret = rt_mutex_timed_lock(&q.pi_state->pi_mutex, to, 1); else { ret = rt_mutex_trylock(&q.pi_state->pi_mutex); /* Fixup the trylock return value: */ ret = ret ? 0 : -EWOULDBLOCK; } spin_lock(q.lock_ptr); /* * Fixup the pi_state owner and possibly acquire the lock if we * haven't already. */ res = fixup_owner(uaddr, &q, !ret); /* * If fixup_owner() returned an error, proprogate that. If it acquired * the lock, clear our -ETIMEDOUT or -EINTR. */ if (res) ret = (res < 0) ? res : 0; /* * If fixup_owner() faulted and was unable to handle the fault, unlock * it and return the fault to userspace. */ if (ret && (rt_mutex_owner(&q.pi_state->pi_mutex) == current)) rt_mutex_unlock(&q.pi_state->pi_mutex); /* Unqueue and drop the lock */ unqueue_me_pi(&q); goto out_put_key; out_unlock_put_key: queue_unlock(&q, hb); out_put_key: put_futex_key(&q.key); out: if (to) destroy_hrtimer_on_stack(&to->timer); return ret != -EINTR ? ret : -ERESTARTNOINTR; uaddr_faulted: queue_unlock(&q, hb); ret = fault_in_user_writeable(uaddr); if (ret) goto out_put_key; if (!(flags & FLAGS_SHARED)) goto retry_private; put_futex_key(&q.key); goto retry; } /* * Userspace attempted a TID -> 0 atomic transition, and failed. * This is the in-kernel slowpath: we look up the PI state (if any), * and do the rt-mutex unlock. */ static int futex_unlock_pi(u32 __user *uaddr, unsigned int flags) { struct futex_hash_bucket *hb; struct futex_q *this, *next; struct plist_head *head; union futex_key key = FUTEX_KEY_INIT; u32 uval, vpid = task_pid_vnr(current); int ret; retry: if (get_user(uval, uaddr)) return -EFAULT; /* * We release only a lock we actually own: */ if ((uval & FUTEX_TID_MASK) != vpid) return -EPERM; ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &key, VERIFY_WRITE); if (unlikely(ret != 0)) goto out; hb = hash_futex(&key); spin_lock(&hb->lock); /* * To avoid races, try to do the TID -> 0 atomic transition * again. If it succeeds then we can return without waking * anyone else up: */ if (!(uval & FUTEX_OWNER_DIED) && cmpxchg_futex_value_locked(&uval, uaddr, vpid, 0)) goto pi_faulted; /* * Rare case: we managed to release the lock atomically, * no need to wake anyone else up: */ if (unlikely(uval == vpid)) goto out_unlock; /* * Ok, other tasks may need to be woken up - check waiters * and do the wakeup if necessary: */ head = &hb->chain; plist_for_each_entry_safe(this, next, head, list) { if (!match_futex (&this->key, &key)) continue; ret = wake_futex_pi(uaddr, uval, this); /* * The atomic access to the futex value * generated a pagefault, so retry the * user-access and the wakeup: */ if (ret == -EFAULT) goto pi_faulted; goto out_unlock; } /* * No waiters - kernel unlocks the futex: */ if (!(uval & FUTEX_OWNER_DIED)) { ret = unlock_futex_pi(uaddr, uval); if (ret == -EFAULT) goto pi_faulted; } out_unlock: spin_unlock(&hb->lock); put_futex_key(&key); out: return ret; pi_faulted: spin_unlock(&hb->lock); put_futex_key(&key); ret = fault_in_user_writeable(uaddr); if (!ret) goto retry; return ret; } /** * handle_early_requeue_pi_wakeup() - Detect early wakeup on the initial futex * @hb: the hash_bucket futex_q was original enqueued on * @q: the futex_q woken while waiting to be requeued * @key2: the futex_key of the requeue target futex * @timeout: the timeout associated with the wait (NULL if none) * * Detect if the task was woken on the initial futex as opposed to the requeue * target futex. If so, determine if it was a timeout or a signal that caused * the wakeup and return the appropriate error code to the caller. Must be * called with the hb lock held. * * Returns * 0 - no early wakeup detected * <0 - -ETIMEDOUT or -ERESTARTNOINTR */ static inline int handle_early_requeue_pi_wakeup(struct futex_hash_bucket *hb, struct futex_q *q, union futex_key *key2, struct hrtimer_sleeper *timeout) { int ret = 0; /* * With the hb lock held, we avoid races while we process the wakeup. * We only need to hold hb (and not hb2) to ensure atomicity as the * wakeup code can't change q.key from uaddr to uaddr2 if we hold hb. * It can't be requeued from uaddr2 to something else since we don't * support a PI aware source futex for requeue. */ if (!match_futex(&q->key, key2)) { WARN_ON(q->lock_ptr && (&hb->lock != q->lock_ptr)); /* * We were woken prior to requeue by a timeout or a signal. * Unqueue the futex_q and determine which it was. */ plist_del(&q->list, &hb->chain); /* Handle spurious wakeups gracefully */ ret = -EWOULDBLOCK; if (timeout && !timeout->task) ret = -ETIMEDOUT; else if (signal_pending(current)) ret = -ERESTARTNOINTR; } return ret; } /** * futex_wait_requeue_pi() - Wait on uaddr and take uaddr2 * @uaddr: the futex we initially wait on (non-pi) * @flags: futex flags (FLAGS_SHARED, FLAGS_CLOCKRT, etc.), they must be * the same type, no requeueing from private to shared, etc. * @val: the expected value of uaddr * @abs_time: absolute timeout * @bitset: 32 bit wakeup bitset set by userspace, defaults to all * @clockrt: whether to use CLOCK_REALTIME (1) or CLOCK_MONOTONIC (0) * @uaddr2: the pi futex we will take prior to returning to user-space * * The caller will wait on uaddr and will be requeued by futex_requeue() to * uaddr2 which must be PI aware and unique from uaddr. Normal wakeup will wake * on uaddr2 and complete the acquisition of the rt_mutex prior to returning to * userspace. This ensures the rt_mutex maintains an owner when it has waiters; * without one, the pi logic would not know which task to boost/deboost, if * there was a need to. * * We call schedule in futex_wait_queue_me() when we enqueue and return there * via the following: * 1) wakeup on uaddr2 after an atomic lock acquisition by futex_requeue() * 2) wakeup on uaddr2 after a requeue * 3) signal * 4) timeout * * If 3, cleanup and return -ERESTARTNOINTR. * * If 2, we may then block on trying to take the rt_mutex and return via: * 5) successful lock * 6) signal * 7) timeout * 8) other lock acquisition failure * * If 6, return -EWOULDBLOCK (restarting the syscall would do the same). * * If 4 or 7, we cleanup and return with -ETIMEDOUT. * * Returns: * 0 - On success * <0 - On error */ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags, u32 val, ktime_t *abs_time, u32 bitset, u32 __user *uaddr2) { struct hrtimer_sleeper timeout, *to = NULL; struct rt_mutex_waiter rt_waiter; struct rt_mutex *pi_mutex = NULL; struct futex_hash_bucket *hb; union futex_key key2 = FUTEX_KEY_INIT; struct futex_q q = futex_q_init; int res, ret; if (uaddr == uaddr2) return -EINVAL; if (!bitset) return -EINVAL; if (abs_time) { to = &timeout; hrtimer_init_on_stack(&to->timer, (flags & FLAGS_CLOCKRT) ? CLOCK_REALTIME : CLOCK_MONOTONIC, HRTIMER_MODE_ABS); hrtimer_init_sleeper(to, current); hrtimer_set_expires_range_ns(&to->timer, *abs_time, current->timer_slack_ns); } /* * The waiter is allocated on our stack, manipulated by the requeue * code while we sleep on uaddr. */ debug_rt_mutex_init_waiter(&rt_waiter); rt_waiter.task = NULL; ret = get_futex_key(uaddr2, flags & FLAGS_SHARED, &key2, VERIFY_WRITE); if (unlikely(ret != 0)) goto out; q.bitset = bitset; q.rt_waiter = &rt_waiter; q.requeue_pi_key = &key2; /* * Prepare to wait on uaddr. On success, increments q.key (key1) ref * count. */ ret = futex_wait_setup(uaddr, val, flags, &q, &hb); if (ret) goto out_key2; /* Queue the futex_q, drop the hb lock, wait for wakeup. */ futex_wait_queue_me(hb, &q, to); spin_lock(&hb->lock); ret = handle_early_requeue_pi_wakeup(hb, &q, &key2, to); spin_unlock(&hb->lock); if (ret) goto out_put_keys; /* * In order for us to be here, we know our q.key == key2, and since * we took the hb->lock above, we also know that futex_requeue() has * completed and we no longer have to concern ourselves with a wakeup * race with the atomic proxy lock acquisition by the requeue code. The * futex_requeue dropped our key1 reference and incremented our key2 * reference count. */ /* Check if the requeue code acquired the second futex for us. */ if (!q.rt_waiter) { /* * Got the lock. We might not be the anticipated owner if we * did a lock-steal - fix up the PI-state in that case. */ if (q.pi_state && (q.pi_state->owner != current)) { spin_lock(q.lock_ptr); ret = fixup_pi_state_owner(uaddr2, &q, current); spin_unlock(q.lock_ptr); } } else { /* * We have been woken up by futex_unlock_pi(), a timeout, or a * signal. futex_unlock_pi() will not destroy the lock_ptr nor * the pi_state. */ WARN_ON(!q.pi_state); pi_mutex = &q.pi_state->pi_mutex; ret = rt_mutex_finish_proxy_lock(pi_mutex, to, &rt_waiter, 1); debug_rt_mutex_free_waiter(&rt_waiter); spin_lock(q.lock_ptr); /* * Fixup the pi_state owner and possibly acquire the lock if we * haven't already. */ res = fixup_owner(uaddr2, &q, !ret); /* * If fixup_owner() returned an error, proprogate that. If it * acquired the lock, clear -ETIMEDOUT or -EINTR. */ if (res) ret = (res < 0) ? res : 0; /* Unqueue and drop the lock. */ unqueue_me_pi(&q); } /* * If fixup_pi_state_owner() faulted and was unable to handle the * fault, unlock the rt_mutex and return the fault to userspace. */ if (ret == -EFAULT) { if (pi_mutex && rt_mutex_owner(pi_mutex) == current) rt_mutex_unlock(pi_mutex); } else if (ret == -EINTR) { /* * We've already been requeued, but cannot restart by calling * futex_lock_pi() directly. We could restart this syscall, but * it would detect that the user space "val" changed and return * -EWOULDBLOCK. Save the overhead of the restart and return * -EWOULDBLOCK directly. */ ret = -EWOULDBLOCK; } out_put_keys: put_futex_key(&q.key); out_key2: put_futex_key(&key2); out: if (to) { hrtimer_cancel(&to->timer); destroy_hrtimer_on_stack(&to->timer); } return ret; } /* * Support for robust futexes: the kernel cleans up held futexes at * thread exit time. * * Implementation: user-space maintains a per-thread list of locks it * is holding. Upon do_exit(), the kernel carefully walks this list, * and marks all locks that are owned by this thread with the * FUTEX_OWNER_DIED bit, and wakes up a waiter (if any). The list is * always manipulated with the lock held, so the list is private and * per-thread. Userspace also maintains a per-thread 'list_op_pending' * field, to allow the kernel to clean up if the thread dies after * acquiring the lock, but just before it could have added itself to * the list. There can only be one such pending lock. */ /** * sys_set_robust_list() - Set the robust-futex list head of a task * @head: pointer to the list-head * @len: length of the list-head, as userspace expects */ SYSCALL_DEFINE2(set_robust_list, struct robust_list_head __user *, head, size_t, len) { if (!futex_cmpxchg_enabled) return -ENOSYS; /* * The kernel knows only one size for now: */ if (unlikely(len != sizeof(*head))) return -EINVAL; current->robust_list = head; return 0; } /** * sys_get_robust_list() - Get the robust-futex list head of a task * @pid: pid of the process [zero for current task] * @head_ptr: pointer to a list-head pointer, the kernel fills it in * @len_ptr: pointer to a length field, the kernel fills in the header size */ SYSCALL_DEFINE3(get_robust_list, int, pid, struct robust_list_head __user * __user *, head_ptr, size_t __user *, len_ptr) { struct robust_list_head __user *head; unsigned long ret; struct task_struct *p; if (!futex_cmpxchg_enabled) return -ENOSYS; WARN_ONCE(1, "deprecated: get_robust_list will be deleted in 2013.\n"); rcu_read_lock(); ret = -ESRCH; if (!pid) p = current; else { p = find_task_by_vpid(pid); if (!p) goto err_unlock; } ret = -EPERM; if (!ptrace_may_access(p, PTRACE_MODE_READ)) goto err_unlock; head = p->robust_list; rcu_read_unlock(); if (put_user(sizeof(*head), len_ptr)) return -EFAULT; return put_user(head, head_ptr); err_unlock: rcu_read_unlock(); return ret; } /* * Process a futex-list entry, check whether it's owned by the * dying task, and do notification if so: */ int handle_futex_death(u32 __user *uaddr, struct task_struct *curr, int pi) { u32 uval, uninitialized_var(nval), mval; retry: if (get_user(uval, uaddr)) return -1; if ((uval & FUTEX_TID_MASK) == task_pid_vnr(curr)) { /* * Ok, this dying thread is truly holding a futex * of interest. Set the OWNER_DIED bit atomically * via cmpxchg, and if the value had FUTEX_WAITERS * set, wake up a waiter (if any). (We have to do a * futex_wake() even if OWNER_DIED is already set - * to handle the rare but possible case of recursive * thread-death.) The rest of the cleanup is done in * userspace. */ mval = (uval & FUTEX_WAITERS) | FUTEX_OWNER_DIED; /* * We are not holding a lock here, but we want to have * the pagefault_disable/enable() protection because * we want to handle the fault gracefully. If the * access fails we try to fault in the futex with R/W * verification via get_user_pages. get_user() above * does not guarantee R/W access. If that fails we * give up and leave the futex locked. */ if (cmpxchg_futex_value_locked(&nval, uaddr, uval, mval)) { if (fault_in_user_writeable(uaddr)) return -1; goto retry; } if (nval != uval) goto retry; /* * Wake robust non-PI futexes here. The wakeup of * PI futexes happens in exit_pi_state(): */ if (!pi && (uval & FUTEX_WAITERS)) futex_wake(uaddr, 1, 1, FUTEX_BITSET_MATCH_ANY); } return 0; } /* * Fetch a robust-list pointer. Bit 0 signals PI futexes: */ static inline int fetch_robust_entry(struct robust_list __user **entry, struct robust_list __user * __user *head, unsigned int *pi) { unsigned long uentry; if (get_user(uentry, (unsigned long __user *)head)) return -EFAULT; *entry = (void __user *)(uentry & ~1UL); *pi = uentry & 1; return 0; } /* * Walk curr->robust_list (very carefully, it's a userspace list!) * and mark any locks found there dead, and notify any waiters. * * We silently return on any sign of list-walking problem. */ void exit_robust_list(struct task_struct *curr) { struct robust_list_head __user *head = curr->robust_list; struct robust_list __user *entry, *next_entry, *pending; unsigned int limit = ROBUST_LIST_LIMIT, pi, pip; unsigned int uninitialized_var(next_pi); unsigned long futex_offset; int rc; if (!futex_cmpxchg_enabled) return; /* * Fetch the list head (which was registered earlier, via * sys_set_robust_list()): */ if (fetch_robust_entry(&entry, &head->list.next, &pi)) return; /* * Fetch the relative futex offset: */ if (get_user(futex_offset, &head->futex_offset)) return; /* * Fetch any possibly pending lock-add first, and handle it * if it exists: */ if (fetch_robust_entry(&pending, &head->list_op_pending, &pip)) return; next_entry = NULL; /* avoid warning with gcc */ while (entry != &head->list) { /* * Fetch the next entry in the list before calling * handle_futex_death: */ rc = fetch_robust_entry(&next_entry, &entry->next, &next_pi); /* * A pending lock might already be on the list, so * don't process it twice: */ if (entry != pending) if (handle_futex_death((void __user *)entry + futex_offset, curr, pi)) return; if (rc) return; entry = next_entry; pi = next_pi; /* * Avoid excessively long or circular lists: */ if (!--limit) break; cond_resched(); } if (pending) handle_futex_death((void __user *)pending + futex_offset, curr, pip); } long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout, u32 __user *uaddr2, u32 val2, u32 val3) { int cmd = op & FUTEX_CMD_MASK; unsigned int flags = 0; if (!(op & FUTEX_PRIVATE_FLAG)) flags |= FLAGS_SHARED; if (op & FUTEX_CLOCK_REALTIME) { flags |= FLAGS_CLOCKRT; if (cmd != FUTEX_WAIT_BITSET && cmd != FUTEX_WAIT_REQUEUE_PI) return -ENOSYS; } switch (cmd) { case FUTEX_LOCK_PI: case FUTEX_UNLOCK_PI: case FUTEX_TRYLOCK_PI: case FUTEX_WAIT_REQUEUE_PI: case FUTEX_CMP_REQUEUE_PI: if (!futex_cmpxchg_enabled) return -ENOSYS; } switch (cmd) { case FUTEX_WAIT: val3 = FUTEX_BITSET_MATCH_ANY; case FUTEX_WAIT_BITSET: return futex_wait(uaddr, flags, val, timeout, val3); case FUTEX_WAKE: val3 = FUTEX_BITSET_MATCH_ANY; case FUTEX_WAKE_BITSET: return futex_wake(uaddr, flags, val, val3); case FUTEX_REQUEUE: return futex_requeue(uaddr, flags, uaddr2, val, val2, NULL, 0); case FUTEX_CMP_REQUEUE: return futex_requeue(uaddr, flags, uaddr2, val, val2, &val3, 0); case FUTEX_WAKE_OP: return futex_wake_op(uaddr, flags, uaddr2, val, val2, val3); case FUTEX_LOCK_PI: return futex_lock_pi(uaddr, flags, val, timeout, 0); case FUTEX_UNLOCK_PI: return futex_unlock_pi(uaddr, flags); case FUTEX_TRYLOCK_PI: return futex_lock_pi(uaddr, flags, 0, timeout, 1); case FUTEX_WAIT_REQUEUE_PI: val3 = FUTEX_BITSET_MATCH_ANY; return futex_wait_requeue_pi(uaddr, flags, val, timeout, val3, uaddr2); case FUTEX_CMP_REQUEUE_PI: return futex_requeue(uaddr, flags, uaddr2, val, val2, &val3, 1); } return -ENOSYS; } SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val, struct timespec __user *, utime, u32 __user *, uaddr2, u32, val3) { struct timespec ts; ktime_t t, *tp = NULL; u32 val2 = 0; int cmd = op & FUTEX_CMD_MASK; if (utime && (cmd == FUTEX_WAIT || cmd == FUTEX_LOCK_PI || cmd == FUTEX_WAIT_BITSET || cmd == FUTEX_WAIT_REQUEUE_PI)) { if (copy_from_user(&ts, utime, sizeof(ts)) != 0) return -EFAULT; if (!timespec_valid(&ts)) return -EINVAL; t = timespec_to_ktime(ts); if (cmd == FUTEX_WAIT) t = ktime_add_safe(ktime_get(), t); tp = &t; } /* * requeue parameter in 'utime' if cmd == FUTEX_*_REQUEUE_*. * number of waiters to wake in 'utime' if cmd == FUTEX_WAKE_OP. */ if (cmd == FUTEX_REQUEUE || cmd == FUTEX_CMP_REQUEUE || cmd == FUTEX_CMP_REQUEUE_PI || cmd == FUTEX_WAKE_OP) val2 = (u32) (unsigned long) utime; return do_futex(uaddr, op, val, tp, uaddr2, val2, val3); } static int __init futex_init(void) { u32 curval; int i; /* * This will fail and we want it. Some arch implementations do * runtime detection of the futex_atomic_cmpxchg_inatomic() * functionality. We want to know that before we call in any * of the complex code paths. Also we want to prevent * registration of robust lists in that case. NULL is * guaranteed to fault and we get -EFAULT on functional * implementation, the non-functional ones will return * -ENOSYS. */ if (cmpxchg_futex_value_locked(&curval, NULL, 0, 0) == -EFAULT) futex_cmpxchg_enabled = 1; for (i = 0; i < ARRAY_SIZE(futex_queues); i++) { plist_head_init(&futex_queues[i].chain); spin_lock_init(&futex_queues[i].lock); } return 0; } __initcall(futex_init);