/* * Percpu refcounts: * (C) 2012 Google, Inc. * Author: Kent Overstreet <koverstreet@google.com> * * This implements a refcount with similar semantics to atomic_t - atomic_inc(), * atomic_dec_and_test() - but percpu. * * There's one important difference between percpu refs and normal atomic_t * refcounts; you have to keep track of your initial refcount, and then when you * start shutting down you call percpu_ref_kill() _before_ dropping the initial * refcount. * * The refcount will have a range of 0 to ((1U << 31) - 1), i.e. one bit less * than an atomic_t - this is because of the way shutdown works, see * percpu_ref_kill()/PCPU_COUNT_BIAS. * * Before you call percpu_ref_kill(), percpu_ref_put() does not check for the * refcount hitting 0 - it can't, if it was in percpu mode. percpu_ref_kill() * puts the ref back in single atomic_t mode, collecting the per cpu refs and * issuing the appropriate barriers, and then marks the ref as shutting down so * that percpu_ref_put() will check for the ref hitting 0. After it returns, * it's safe to drop the initial ref. * * USAGE: * * See fs/aio.c for some example usage; it's used there for struct kioctx, which * is created when userspaces calls io_setup(), and destroyed when userspace * calls io_destroy() or the process exits. * * In the aio code, kill_ioctx() is called when we wish to destroy a kioctx; it * calls percpu_ref_kill(), then hlist_del_rcu() and sychronize_rcu() to remove * the kioctx from the proccess's list of kioctxs - after that, there can't be * any new users of the kioctx (from lookup_ioctx()) and it's then safe to drop * the initial ref with percpu_ref_put(). * * Code that does a two stage shutdown like this often needs some kind of * explicit synchronization to ensure the initial refcount can only be dropped * once - percpu_ref_kill() does this for you, it returns true once and false if * someone else already called it. The aio code uses it this way, but it's not * necessary if the code has some other mechanism to synchronize teardown. * around. */ #ifndef _LINUX_PERCPU_REFCOUNT_H #define _LINUX_PERCPU_REFCOUNT_H #include <linux/atomic.h> #include <linux/kernel.h> #include <linux/percpu.h> #include <linux/rcupdate.h> struct percpu_ref; typedef void (percpu_ref_func_t)(struct percpu_ref *); struct percpu_ref { atomic_t count; /* * The low bit of the pointer indicates whether the ref is in percpu * mode; if set, then get/put will manipulate the atomic_t (this is a * hack because we need to keep the pointer around for * percpu_ref_kill_rcu()) */ unsigned __percpu *pcpu_count; percpu_ref_func_t *release; percpu_ref_func_t *confirm_kill; struct rcu_head rcu; }; int __must_check percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release); void percpu_ref_cancel_init(struct percpu_ref *ref); void percpu_ref_kill_and_confirm(struct percpu_ref *ref, percpu_ref_func_t *confirm_kill); /** * percpu_ref_kill - drop the initial ref * @ref: percpu_ref to kill * * Must be used to drop the initial ref on a percpu refcount; must be called * precisely once before shutdown. * * Puts @ref in non percpu mode, then does a call_rcu() before gathering up the * percpu counters and dropping the initial ref. */ static inline void percpu_ref_kill(struct percpu_ref *ref) { return percpu_ref_kill_and_confirm(ref, NULL); } #define PCPU_STATUS_BITS 2 #define PCPU_STATUS_MASK ((1 << PCPU_STATUS_BITS) - 1) #define PCPU_REF_PTR 0 #define PCPU_REF_DEAD 1 #define REF_STATUS(count) (((unsigned long) count) & PCPU_STATUS_MASK) /** * percpu_ref_get - increment a percpu refcount * @ref: percpu_ref to get * * Analagous to atomic_inc(). */ static inline void percpu_ref_get(struct percpu_ref *ref) { unsigned __percpu *pcpu_count; rcu_read_lock_sched(); pcpu_count = ACCESS_ONCE(ref->pcpu_count); if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR)) __this_cpu_inc(*pcpu_count); else atomic_inc(&ref->count); rcu_read_unlock_sched(); } /** * percpu_ref_tryget - try to increment a percpu refcount * @ref: percpu_ref to try-get * * Increment a percpu refcount unless it has already been killed. Returns * %true on success; %false on failure. * * Completion of percpu_ref_kill() in itself doesn't guarantee that tryget * will fail. For such guarantee, percpu_ref_kill_and_confirm() should be * used. After the confirm_kill callback is invoked, it's guaranteed that * no new reference will be given out by percpu_ref_tryget(). */ static inline bool percpu_ref_tryget(struct percpu_ref *ref) { unsigned __percpu *pcpu_count; int ret = false; rcu_read_lock_sched(); pcpu_count = ACCESS_ONCE(ref->pcpu_count); if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR)) { __this_cpu_inc(*pcpu_count); ret = true; } rcu_read_unlock_sched(); return ret; } /** * percpu_ref_put - decrement a percpu refcount * @ref: percpu_ref to put * * Decrement the refcount, and if 0, call the release function (which was passed * to percpu_ref_init()) */ static inline void percpu_ref_put(struct percpu_ref *ref) { unsigned __percpu *pcpu_count; rcu_read_lock_sched(); pcpu_count = ACCESS_ONCE(ref->pcpu_count); if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR)) __this_cpu_dec(*pcpu_count); else if (unlikely(atomic_dec_and_test(&ref->count))) ref->release(ref); rcu_read_unlock_sched(); } #endif