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- /* SPDX-License-Identifier: GPL-2.0 */
- /*
- * Percpu refcounts:
- * (C) 2012 Google, Inc.
- * Author: Kent Overstreet <[email protected]>
- *
- * 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
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