| /* |
| * 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 synchronize_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. |
| */ |
| unsigned long pcpu_count_ptr; |
| 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_reinit(struct percpu_ref *ref); |
| void percpu_ref_exit(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_REF_DEAD 1 |
| |
| /* |
| * 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->pcpu_count is not NULL. |
| */ |
| static inline bool __pcpu_ref_alive(struct percpu_ref *ref, |
| unsigned __percpu **pcpu_countp) |
| { |
| unsigned long pcpu_ptr = ACCESS_ONCE(ref->pcpu_count_ptr); |
| |
| /* paired with smp_store_release() in percpu_ref_reinit() */ |
| smp_read_barrier_depends(); |
| |
| if (unlikely(pcpu_ptr & PCPU_REF_DEAD)) |
| return false; |
| |
| *pcpu_countp = (unsigned __percpu *)pcpu_ptr; |
| return true; |
| } |
| |
| /** |
| * 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(); |
| |
| if (__pcpu_ref_alive(ref, &pcpu_count)) |
| 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 its count already reached zero. |
| * Returns %true on success; %false on failure. |
| * |
| * The caller is responsible for ensuring that @ref stays accessible. |
| */ |
| static inline bool percpu_ref_tryget(struct percpu_ref *ref) |
| { |
| unsigned __percpu *pcpu_count; |
| int ret = false; |
| |
| rcu_read_lock_sched(); |
| |
| if (__pcpu_ref_alive(ref, &pcpu_count)) { |
| this_cpu_inc(*pcpu_count); |
| ret = true; |
| } else { |
| ret = atomic_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 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(). |
| * |
| * The caller is responsible for ensuring that @ref stays accessible. |
| */ |
| static inline bool percpu_ref_tryget_live(struct percpu_ref *ref) |
| { |
| unsigned __percpu *pcpu_count; |
| int ret = false; |
| |
| rcu_read_lock_sched(); |
| |
| if (__pcpu_ref_alive(ref, &pcpu_count)) { |
| 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(); |
| |
| if (__pcpu_ref_alive(ref, &pcpu_count)) |
| this_cpu_dec(*pcpu_count); |
| else if (unlikely(atomic_dec_and_test(&ref->count))) |
| ref->release(ref); |
| |
| rcu_read_unlock_sched(); |
| } |
| |
| /** |
| * percpu_ref_is_zero - test whether a percpu refcount reached zero |
| * @ref: percpu_ref to test |
| * |
| * Returns %true if @ref reached zero. |
| */ |
| static inline bool percpu_ref_is_zero(struct percpu_ref *ref) |
| { |
| unsigned __percpu *pcpu_count; |
| |
| if (__pcpu_ref_alive(ref, &pcpu_count)) |
| return false; |
| return !atomic_read(&ref->count); |
| } |
| |
| #endif |