From: Kent Overstreet <kmo@daterainc.com>
To: Tejun Heo <tj@kernel.org>
Cc: Christoph Lameter <cl@linux-foundation.org>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2 percpu/for-3.20] percpu_ref: remove unnecessary ACCESS_ONCE() in percpu_ref_tryget_live()
Date: Tue, 6 Jan 2015 18:21:28 -0800 [thread overview]
Message-ID: <20150107022128.GC2433@kmo-pixel> (raw)
In-Reply-To: <20150106152347.GD3077@htj.dyndns.org>
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@daterainc.com>
prev parent reply other threads:[~2015-01-07 2:18 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-01-06 15:23 [PATCH 1/2 percpu/for-3.20] percpu_ref: remove unnecessary ACCESS_ONCE() in percpu_ref_tryget_live() Tejun Heo
2015-01-06 15:24 ` [PATCH 2/2 percpu/for-3.20] percpu_ref: implement percpu_ref_is_dying() Tejun Heo
2015-01-07 2:21 ` Kent Overstreet
2015-01-07 2:21 ` Kent Overstreet [this message]
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