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Commit 6810e4a3 authored by Tejun Heo's avatar Tejun Heo
Browse files

percpu_ref: remove unnecessary ACCESS_ONCE() in percpu_ref_tryget_live() 



__ref_is_percpu() needs the implied ACCESS_ONCE() in
lockless_dereference() on @ref->percpu_count_ptr because the value 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.

percpu_ref_tryget_live() also uses ACCESS_ONCE() to test
__PERCPU_REF_DEAD; however, there's no reason for this.  I just copied
ACCESS_ONCE() usage blindly from __ref_is_percpu().  All it does is
confusing people trying to understand what's going on.

This patch removes the unnecessary ACCESS_ONCE() usage from
percpu_ref_tryget_live() and adds a comment explaining why
__ref_is_percpu() needs it.

Signed-off-by: default avatarTejun Heo <tj@kernel.org>
Cc: Kent Overstreet <kmo@daterainc.com>
parent 97bf6af1

include/linux/percpu-refcount.h

+17 −3
Original line number Original line Diff line number Diff line
@@ -128,8 +128,22 @@ static inline void percpu_ref_kill(struct percpu_ref *ref) 
static inline bool __ref_is_percpu(struct percpu_ref *ref,

static inline bool __ref_is_percpu(struct percpu_ref *ref,

					  unsigned long __percpu **percpu_countp)

					  unsigned long __percpu **percpu_countp)

{

{

	/* paired with smp_store_release() in percpu_ref_reinit() */

	unsigned long percpu_ptr;

	unsigned long percpu_ptr = lockless_dereference(ref->percpu_count_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

	 * ACCESS_ONCE() is required when fetching it.

	 *

	 * Also, we need a data dependency barrier to be paired with

	 * smp_store_release() in __percpu_ref_switch_to_percpu().

	 *

	 * Use lockless deref which contains both.

	 */

	percpu_ptr = lockless_dereference(ref->percpu_count_ptr);





	/*

	/*

	 * Theoretically, the following could test just ATOMIC; however,

	 * Theoretically, the following could test just ATOMIC; however,
@@ -233,7 +247,7 @@ static inline bool percpu_ref_tryget_live(struct percpu_ref *ref) 
	if (__ref_is_percpu(ref, &percpu_count)) {

	if (__ref_is_percpu(ref, &percpu_count)) {

		this_cpu_inc(*percpu_count);

		this_cpu_inc(*percpu_count);

		ret = true;

		ret = true;

	} else if (!(ACCESS_ONCE(ref->percpu_count_ptr) & __PERCPU_REF_DEAD)) {

	} else if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD)) {

		ret = atomic_long_inc_not_zero(&ref->count);

		ret = atomic_long_inc_not_zero(&ref->count);

	}

	}