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Commit 2d722782 authored by Tejun Heo's avatar Tejun Heo
Browse files

percpu-refcount: implement percpu_ref_reinit() and percpu_ref_is_zero() 



Now that explicit invocation of percpu_ref_exit() is necessary to free
the percpu counter, we can implement percpu_ref_reinit() which
reinitializes a released percpu_ref.  This can be used implement
scalable gating switch which can be drained and then re-opened without
worrying about memory allocation failures.

percpu_ref_is_zero() is added to be used in a sanity check in
percpu_ref_exit().  As this function will be useful for other purposes
too, make it a public interface.

v2: Use smp_read_barrier_depends() instead of smp_load_acquire().  We
    only need data dep barrier and smp_load_acquire() is stronger and
    heavier on some archs.  Spotted by Lai Jiangshan.

Signed-off-by: default avatarTejun Heo <tj@kernel.org>
Cc: Kent Overstreet <kmo@daterainc.com>
Cc: Christoph Lameter <cl@linux-foundation.org>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
parent 9a1049da

include/linux/percpu-refcount.h

+19 −0
Original line number Diff line number Diff line
@@ -67,6 +67,7 @@ struct percpu_ref { 


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);
@@ -99,6 +100,9 @@ static inline bool __pcpu_ref_alive(struct percpu_ref *ref, 
{

	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;
@@ -206,4 +210,19 @@ static inline void percpu_ref_put(struct percpu_ref *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

lib/percpu-refcount.c

+35 −0
Original line number Diff line number Diff line
@@ -60,6 +60,41 @@ int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release) 
}

EXPORT_SYMBOL_GPL(percpu_ref_init);



/**

 * percpu_ref_reinit - re-initialize a percpu refcount

 * @ref: perpcu_ref to re-initialize

 *

 * Re-initialize @ref so that it's in the same state as when it finished

 * percpu_ref_init().  @ref must have been initialized successfully, killed

 * and reached 0 but not exited.

 *

 * Note that percpu_ref_tryget[_live]() are safe to perform on @ref while

 * this function is in progress.

 */

void percpu_ref_reinit(struct percpu_ref *ref)

{

	unsigned __percpu *pcpu_count = pcpu_count_ptr(ref);

	int cpu;



	BUG_ON(!pcpu_count);

	WARN_ON(!percpu_ref_is_zero(ref));



	atomic_set(&ref->count, 1 + PCPU_COUNT_BIAS);



	/*

	 * Restore per-cpu operation.  smp_store_release() is paired with

	 * smp_read_barrier_depends() in __pcpu_ref_alive() and guarantees

	 * that the zeroing is visible to all percpu accesses which can see

	 * the following PCPU_REF_DEAD clearing.

	 */

	for_each_possible_cpu(cpu)

		*per_cpu_ptr(pcpu_count, cpu) = 0;



	smp_store_release(&ref->pcpu_count_ptr,

			  ref->pcpu_count_ptr & ~PCPU_REF_DEAD);

}

EXPORT_SYMBOL_GPL(percpu_ref_reinit);



/**

 * percpu_ref_exit - undo percpu_ref_init()

 * @ref: percpu_ref to exit