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/* SPDX-License-Identifier: GPL-2.0 / / * 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()/PERCPU_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 * removes the kioctx from the proccess's table of kioctxs and kills percpu_ref. * 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(). * * Note that the free path, free_ioctx(), needs to go through explicit call_rcu() * to synchronize with RCU protected lookup_ioctx(). percpu_ref operations don't * imply RCU grace periods of any kind and if a user wants to combine percpu_ref * with RCU protection, it must be done explicitly. * * 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> #include <linux/gfp.h> struct percpu_ref; typedef void (percpu_ref_func_t)(struct percpu_ref ); /* flags set in the lower bits of percpu_ref->percpu_count_ptr / enum { __PERCPU_REF_ATOMIC = 1LU << 0, / operating in atomic mode / __PERCPU_REF_DEAD = 1LU << 1, / (being) killed / __PERCPU_REF_ATOMIC_DEAD = __PERCPU_REF_ATOMIC \| __PERCPU_REF_DEAD, __PERCPU_REF_FLAG_BITS = 2, }; / @flags for percpu_ref_init() / enum { / * Start w/ ref == 1 in atomic mode. Can be switched to percpu * operation using percpu_ref_switch_to_percpu(). If initialized * with this flag, the ref will stay in atomic mode until * percpu_ref_switch_to_percpu() is invoked on it. * Implies ALLOW_REINIT. / PERCPU_REF_INIT_ATOMIC = 1 << 0, / * Start dead w/ ref == 0 in atomic mode. Must be revived with * percpu_ref_reinit() before used. Implies INIT_ATOMIC and * ALLOW_REINIT. / PERCPU_REF_INIT_DEAD = 1 << 1, / * Allow switching from atomic mode to percpu mode. / PERCPU_REF_ALLOW_REINIT = 1 << 2, }; struct percpu_ref { atomic_long_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. / unsigned long percpu_count_ptr; percpu_ref_func_t release; percpu_ref_func_t confirm_switch; bool force_atomic:1; bool allow_reinit:1; struct rcu_head rcu; }; int __must_check percpu_ref_init(struct percpu_ref ref, percpu_ref_func_t release, unsigned int flags, gfp_t gfp); void percpu_ref_exit(struct percpu_ref ref); void percpu_ref_switch_to_atomic(struct percpu_ref ref, percpu_ref_func_t confirm_switch); void percpu_ref_switch_to_atomic_sync(struct percpu_ref ref); void percpu_ref_switch_to_percpu(struct percpu_ref ref); void percpu_ref_kill_and_confirm(struct percpu_ref ref, percpu_ref_func_t confirm_kill); void percpu_ref_resurrect(struct percpu_ref ref); void percpu_ref_reinit(struct percpu_ref ref); /** * 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. * * Switches @ref into atomic mode before gathering up the percpu counters * and dropping the initial ref. * * There are no implied RCU grace periods between kill and release. / static inline void percpu_ref_kill(struct percpu_ref ref) { percpu_ref_kill_and_confirm(ref, NULL); } /* * Internal helper. Don't use outside percpu-refcount proper. The * function doesn't return the pointer and let the caller test it for NULL * because doing so forces the compiler to generate two conditional * branches as it can't assume that @ref->percpu_count is not NULL. / static inline bool __ref_is_percpu(struct percpu_ref ref, unsigned long __percpu *percpu_countp) { unsigned long percpu_ptr; / * The value of @ref->percpu_count_ptr is tested for * !__PERCPU_REF_ATOMIC, which may be set asynchronously, and then * used as a pointer. If the compiler generates a separate fetch * when using it as a pointer, __PERCPU_REF_ATOMIC may be set in * between contaminating the pointer value, meaning that * READ_ONCE() is required when fetching it. * * The smp_read_barrier_depends() implied by READ_ONCE() pairs * with smp_store_release() in __percpu_ref_switch_to_percpu(). / percpu_ptr = READ_ONCE(ref->percpu_count_ptr); / * Theoretically, the following could test just ATOMIC; however, * then we'd have to mask off DEAD separately as DEAD may be * visible without ATOMIC if we race with percpu_ref_kill(). DEAD * implies ATOMIC anyway. Test them together. / if (unlikely(percpu_ptr & __PERCPU_REF_ATOMIC_DEAD)) return false; percpu_countp = (unsigned long __percpu )percpu_ptr; return true; } /* * percpu_ref_get_many - increment a percpu refcount * @ref: percpu_ref to get * @nr: number of references to get * * Analogous to atomic_long_add(). * * This function is safe to call as long as @ref is between init and exit. / static inline void percpu_ref_get_many(struct percpu_ref ref, unsigned long nr) { unsigned long __percpu percpu_count; rcu_read_lock_sched(); if (__ref_is_percpu(ref, &percpu_count)) this_cpu_add(percpu_count, nr); else atomic_long_add(nr, &ref->count); rcu_read_unlock_sched(); } /** * percpu_ref_get - increment a percpu refcount * @ref: percpu_ref to get * * Analagous to atomic_long_inc(). * * This function is safe to call as long as @ref is between init and exit. / static inline void percpu_ref_get(struct percpu_ref ref) { percpu_ref_get_many(ref, 1); } /** * percpu_ref_tryget - try to increment a percpu refcount * @ref: percpu_ref to try-get * * Increment a percpu refcount unless its count already reached zero. * Returns %true on success; %false on failure. * * This function is safe to call as long as @ref is between init and exit. / static inline bool percpu_ref_tryget(struct percpu_ref ref) { unsigned long __percpu percpu_count; bool ret; rcu_read_lock_sched(); if (__ref_is_percpu(ref, &percpu_count)) { this_cpu_inc(percpu_count); ret = true; } else { ret = atomic_long_inc_not_zero(&ref->count); } rcu_read_unlock_sched(); return ret; } /** * percpu_ref_tryget_live - try to increment a live 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 this * function 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_live(). * * This function is safe to call as long as @ref is between init and exit. / static inline bool percpu_ref_tryget_live(struct percpu_ref ref) { unsigned long __percpu percpu_count; bool ret = false; rcu_read_lock_sched(); if (__ref_is_percpu(ref, &percpu_count)) { this_cpu_inc(percpu_count); ret = true; } else if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD)) { ret = atomic_long_inc_not_zero(&ref->count); } rcu_read_unlock_sched(); return ret; } /** * percpu_ref_put_many - decrement a percpu refcount * @ref: percpu_ref to put * @nr: number of references to put * * Decrement the refcount, and if 0, call the release function (which was passed * to percpu_ref_init()) * * This function is safe to call as long as @ref is between init and exit. / static inline void percpu_ref_put_many(struct percpu_ref ref, unsigned long nr) { unsigned long __percpu percpu_count; rcu_read_lock_sched(); if (__ref_is_percpu(ref, &percpu_count)) this_cpu_sub(percpu_count, nr); else if (unlikely(atomic_long_sub_and_test(nr, &ref->count))) ref->release(ref); rcu_read_unlock_sched(); } /** * 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()) * * This function is safe to call as long as @ref is between init and exit. / static inline void percpu_ref_put(struct percpu_ref ref) { percpu_ref_put_many(ref, 1); } /** * percpu_ref_is_dying - test whether a percpu refcount is dying or dead * @ref: percpu_ref to test * * Returns %true if @ref is dying or dead. * * This function is safe to call as long as @ref is between init and exit * and the caller is responsible for synchronizing against state changes. / static inline bool percpu_ref_is_dying(struct percpu_ref ref) { return ref->percpu_count_ptr & __PERCPU_REF_DEAD; } /** * percpu_ref_is_zero - test whether a percpu refcount reached zero * @ref: percpu_ref to test * * Returns %true if @ref reached zero. * * This function is safe to call as long as @ref is between init and exit. / static inline bool percpu_ref_is_zero(struct percpu_ref ref) { unsigned long __percpu *percpu_count; if (__ref_is_percpu(ref, &percpu_count)) return false; return !atomic_long_read(&ref->count); } #endif

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