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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: Tejun Heo <tj@kernel.org>
Cc: Kent Overstreet <kmo@daterainc.com>
This commit is contained in:
Tejun Heo 2015-01-06 10:26:10 -05:00
parent 97bf6af1f9
commit 6810e4a394
1 changed files with 17 additions and 3 deletions
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20
include/linux/percpu-refcount.h
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@ -128,8 +128,22 @@ static inline void percpu_ref_kill(struct percpu_ref *ref)
static inline bool __ref_is_percpu(struct percpu_ref *ref,
unsigned long __percpu **percpu_countp)
{
/* paired with smp_store_release() in percpu_ref_reinit() */
unsigned long percpu_ptr = lockless_dereference(ref->percpu_count_ptr);
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
* 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,
@ -233,7 +247,7 @@ static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
if (__ref_is_percpu(ref, &percpu_count)) {
this_cpu_inc(*percpu_count);
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);
}