Re: [PATCH 1/2 percpu/for-3.20] percpu_ref: remove unnecessary ACCESS_ONCE() in percpu_ref_tryget_live()

From: Kent Overstreet
Date: Tue Jan 06 2015 - 21:19:04 EST


On Tue, Jan 06, 2015 at 10:23:47AM -0500, Tejun Heo wrote:
> __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.

lockless_dereference() is new - cool! I've been wanting that without even
realizing it.

Acked-by: Kent Overstreet <kmo@xxxxxxxxxxxxx>
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