/* rwsem.c: R/W semaphores: contention handling functions * * Written by David Howells (dhowells@redhat.com). * Derived from arch/i386/kernel/semaphore.c */ #include <linux/rwsem.h> #include <linux/sched.h> #include <linux/init.h> #include <linux/module.h> /* * Initialize an rwsem: */ void __init_rwsem(struct rw_semaphore *sem, const char *name, struct lock_class_key *key) { #ifdef CONFIG_DEBUG_LOCK_ALLOC /* * Make sure we are not reinitializing a held semaphore: */ debug_check_no_locks_freed((void *)sem, sizeof(*sem)); lockdep_init_map(&sem->dep_map, name, key, 0); #endif sem->count = RWSEM_UNLOCKED_VALUE; spin_lock_init(&sem->wait_lock); INIT_LIST_HEAD(&sem->wait_list); } EXPORT_SYMBOL(__init_rwsem); struct rwsem_waiter { struct list_head list; struct task_struct *task; unsigned int flags; #define RWSEM_WAITING_FOR_READ 0x00000001 #define RWSEM_WAITING_FOR_WRITE 0x00000002 }; /* Wake types for __rwsem_do_wake(). Note that RWSEM_WAKE_NO_ACTIVE and * RWSEM_WAKE_READ_OWNED imply that the spinlock must have been kept held * since the rwsem value was observed. */ #define RWSEM_WAKE_ANY 0 /* Wake whatever's at head of wait list */ #define RWSEM_WAKE_NO_ACTIVE 1 /* rwsem was observed with no active thread */ #define RWSEM_WAKE_READ_OWNED 2 /* rwsem was observed to be read owned */ /* * handle the lock release when processes blocked on it that can now run * - if we come here from up_xxxx(), then: * - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed) * - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so) * - there must be someone on the queue * - the spinlock must be held by the caller * - woken process blocks are discarded from the list after having task zeroed * - writers are only woken if downgrading is false */ static struct rw_semaphore * __rwsem_do_wake(struct rw_semaphore *sem, int wake_type) { struct rwsem_waiter *waiter; struct task_struct *tsk; struct list_head *next; signed long oldcount, woken, loop, adjustment; waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list); if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE)) goto readers_only; if (wake_type == RWSEM_WAKE_READ_OWNED) /* Another active reader was observed, so wakeup is not * likely to succeed. Save the atomic op. */ goto out; /* There's a writer at the front of the queue - try to grant it the * write lock. However, we only wake this writer if we can transition * the active part of the count from 0 -> 1 */ adjustment = RWSEM_ACTIVE_WRITE_BIAS; if (waiter->list.next == &sem->wait_list) adjustment -= RWSEM_WAITING_BIAS; try_again_write: oldcount = rwsem_atomic_update(adjustment, sem) - adjustment; if (oldcount & RWSEM_ACTIVE_MASK) /* Someone grabbed the sem already */ goto undo_write; /* We must be careful not to touch 'waiter' after we set ->task = NULL. * It is an allocated on the waiter's stack and may become invalid at * any time after that point (due to a wakeup from another source). */ list_del(&waiter->list); tsk = waiter->task; smp_mb(); waiter->task = NULL; wake_up_process(tsk); put_task_struct(tsk); goto out; readers_only: /* If we come here from up_xxxx(), another thread might have reached * rwsem_down_failed_common() before we acquired the spinlock and * woken up a waiter, making it now active. We prefer to check for * this first in order to not spend too much time with the spinlock * held if we're not going to be able to wake up readers in the end. * * Note that we do not need to update the rwsem count: any writer * trying to acquire rwsem will run rwsem_down_write_failed() due * to the waiting threads and block trying to acquire the spinlock. * * We use a dummy atomic update in order to acquire the cache line * exclusively since we expect to succeed and run the final rwsem * count adjustment pretty soon. */ if (wake_type == RWSEM_WAKE_ANY && rwsem_atomic_update(0, sem) < RWSEM_WAITING_BIAS) /* Someone grabbed the sem for write already */ goto out; /* Grant an infinite number of read locks to the readers at the front * of the queue. Note we increment the 'active part' of the count by * the number of readers before waking any processes up. */ woken = 0; do { woken++; if (waiter->list.next == &sem->wait_list) break; waiter = list_entry(waiter->list.next, struct rwsem_waiter, list); } while (waiter->flags & RWSEM_WAITING_FOR_READ); adjustment = woken * RWSEM_ACTIVE_READ_BIAS; if (waiter->flags & RWSEM_WAITING_FOR_READ) /* hit end of list above */ adjustment -= RWSEM_WAITING_BIAS; rwsem_atomic_add(adjustment, sem); next = sem->wait_list.next; for (loop = woken; loop > 0; loop--) { waiter = list_entry(next, struct rwsem_waiter, list); next = waiter->list.next; tsk = waiter->task; smp_mb(); waiter->task = NULL; wake_up_process(tsk); put_task_struct(tsk); } sem->wait_list.next = next; next->prev = &sem->wait_list; out: return sem; /* undo the change to the active count, but check for a transition * 1->0 */ undo_write: if (rwsem_atomic_update(-adjustment, sem) & RWSEM_ACTIVE_MASK) goto out; goto try_again_write; } /* * wait for a lock to be granted */ static struct rw_semaphore __sched * rwsem_down_failed_common(struct rw_semaphore *sem, unsigned int flags, signed long adjustment) { struct rwsem_waiter waiter; struct task_struct *tsk = current; signed long count; set_task_state(tsk, TASK_UNINTERRUPTIBLE); /* set up my own style of waitqueue */ spin_lock_irq(&sem->wait_lock); waiter.task = tsk; waiter.flags = flags; get_task_struct(tsk); if (list_empty(&sem->wait_list)) adjustment += RWSEM_WAITING_BIAS; list_add_tail(&waiter.list, &sem->wait_list); /* we're now waiting on the lock, but no longer actively locking */ count = rwsem_atomic_update(adjustment, sem); /* If there are no active locks, wake the front queued process(es) up. * * Alternatively, if we're called from a failed down_write(), there * were already threads queued before us and there are no active * writers, the lock must be read owned; so we try to wake any read * locks that were queued ahead of us. */ if (count == RWSEM_WAITING_BIAS) sem = __rwsem_do_wake(sem, RWSEM_WAKE_NO_ACTIVE); else if (count > RWSEM_WAITING_BIAS && adjustment == -RWSEM_ACTIVE_WRITE_BIAS) sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED); spin_unlock_irq(&sem->wait_lock); /* wait to be given the lock */ for (;;) { if (!waiter.task) break; schedule(); set_task_state(tsk, TASK_UNINTERRUPTIBLE); } tsk->state = TASK_RUNNING; return sem; } /* * wait for the read lock to be granted */ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem) { return rwsem_down_failed_common(sem, RWSEM_WAITING_FOR_READ, -RWSEM_ACTIVE_READ_BIAS); } /* * wait for the write lock to be granted */ struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem) { return rwsem_down_failed_common(sem, RWSEM_WAITING_FOR_WRITE, -RWSEM_ACTIVE_WRITE_BIAS); } /* * handle waking up a waiter on the semaphore * - up_read/up_write has decremented the active part of count if we come here */ struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem) { unsigned long flags; spin_lock_irqsave(&sem->wait_lock, flags); /* do nothing if list empty */ if (!list_empty(&sem->wait_list)) sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY); spin_unlock_irqrestore(&sem->wait_lock, flags); return sem; } /* * downgrade a write lock into a read lock * - caller incremented waiting part of count and discovered it still negative * - just wake up any readers at the front of the queue */ struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem) { unsigned long flags; spin_lock_irqsave(&sem->wait_lock, flags); /* do nothing if list empty */ if (!list_empty(&sem->wait_list)) sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED); spin_unlock_irqrestore(&sem->wait_lock, flags); return sem; } EXPORT_SYMBOL(rwsem_down_read_failed); EXPORT_SYMBOL(rwsem_down_write_failed); EXPORT_SYMBOL(rwsem_wake); EXPORT_SYMBOL(rwsem_downgrade_wake);