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

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Message-ID: <20150107022128.GC2433@kmo-pixel>
Date:	Tue, 6 Jan 2015 18:21:28 -0800
From:	Kent Overstreet <kmo@...erainc.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Christoph Lameter <cl@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2 percpu/for-3.20] percpu_ref: remove unnecessary
 ACCESS_ONCE() in percpu_ref_tryget_live()

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@...erainc.com>
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