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Diffstat (limited to 'include/linux/percpu-refcount.h')
-rw-r--r-- | include/linux/percpu-refcount.h | 368 |
1 files changed, 368 insertions, 0 deletions
diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h new file mode 100644 index 000000000..d73a1c08c --- /dev/null +++ b/include/linux/percpu-refcount.h @@ -0,0 +1,368 @@ +/* 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/percpu.h> +#include <linux/rcupdate.h> +#include <linux/types.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_data { + atomic_long_t count; + percpu_ref_func_t *release; + percpu_ref_func_t *confirm_switch; + bool force_atomic:1; + bool allow_reinit:1; + struct rcu_head rcu; + struct percpu_ref *ref; +}; + +struct percpu_ref { + /* + * 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' is often embedded into user structure, and only + * 'percpu_count_ptr' is required in fast path, move other fields + * into 'percpu_ref_data', so we can reduce memory footprint in + * fast path. + */ + struct percpu_ref_data *data; +}; + +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); +bool percpu_ref_is_zero(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 dependency ordering from the 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(); + + if (__ref_is_percpu(ref, &percpu_count)) + this_cpu_add(*percpu_count, nr); + else + atomic_long_add(nr, &ref->data->count); + + rcu_read_unlock(); +} + +/** + * percpu_ref_get - increment a percpu refcount + * @ref: percpu_ref to get + * + * Analogous 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_many - try to increment a percpu refcount + * @ref: percpu_ref to try-get + * @nr: number of references to get + * + * Increment a percpu refcount by @nr 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_many(struct percpu_ref *ref, + unsigned long nr) +{ + unsigned long __percpu *percpu_count; + bool ret; + + rcu_read_lock(); + + if (__ref_is_percpu(ref, &percpu_count)) { + this_cpu_add(*percpu_count, nr); + ret = true; + } else { + ret = atomic_long_add_unless(&ref->data->count, nr, 0); + } + + rcu_read_unlock(); + + return ret; +} + +/** + * 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) +{ + return percpu_ref_tryget_many(ref, 1); +} + +/** + * percpu_ref_tryget_live_rcu - same as percpu_ref_tryget_live() but the + * caller is responsible for taking RCU. + * + * This function is safe to call as long as @ref is between init and exit. + */ +static inline bool percpu_ref_tryget_live_rcu(struct percpu_ref *ref) +{ + unsigned long __percpu *percpu_count; + bool ret = false; + + WARN_ON_ONCE(!rcu_read_lock_held()); + + if (likely(__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->data->count); + } + 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) +{ + bool ret = false; + + rcu_read_lock(); + ret = percpu_ref_tryget_live_rcu(ref); + rcu_read_unlock(); + 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(); + + if (__ref_is_percpu(ref, &percpu_count)) + this_cpu_sub(*percpu_count, nr); + else if (unlikely(atomic_long_sub_and_test(nr, &ref->data->count))) + ref->data->release(ref); + + rcu_read_unlock(); +} + +/** + * 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; +} + +#endif |