From: Shaohua Li <shli@kernel.org>
To: Tejun Heo <tj@kernel.org>
Cc: linux-kernel@vger.kernel.org, Jens Axboe <axboe@fb.com>,
Kent Overstreet <kmo@daterainc.com>
Subject: Re: [PATCH percpu/for-3.18-fixes] percpu-ref: fix DEAD flag contamination of percpu pointer
Date: Sat, 22 Nov 2014 16:47:04 -0800 [thread overview]
Message-ID: <20141123004704.GA2245@kernel.org> (raw)
In-Reply-To: <20141122170602.GE26007@htj.dyndns.org>
On Sat, Nov 22, 2014 at 12:06:02PM -0500, Tejun Heo wrote:
> On Sat, Nov 22, 2014 at 09:04:48AM -0800, Shaohua Li wrote:
> ...
> > > + /*
> > > + * Theoretically, the following could test just ATOMIC; however,
> > > + * then we'd have to mask off DEAD separately as DEAD may be
> > > + * visible without ATOMIC if we race with percpu_ref_kill(). DEAD
> > > + * implies ATOMIC anyway. Test them together.
> > > + */
> > > + if (unlikely(percpu_ptr & __PERCPU_REF_ATOMIC_DEAD))
> > > return false;
> >
> > this sounds not the correct answer. the DEAD/ATOMIC bit can be set by
> > percpu_ref_kill() right after the check.
>
> Yes, but that's why we're fetching @percpu_ptr with ACCESS_ONCE()
> before checking the flags.
Ok, I forgot we cache the percpu_ptr. Yes, this does work. You can add my
signed-off in the patch.
Thanks,
Shaohua
next prev parent reply other threads:[~2014-11-23 0:47 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-11-22 6:48 [PATCH] percpu-ref: correctly get percpu pointer Shaohua Li
2014-11-22 14:22 ` [PATCH percpu/for-3.18-fixes] percpu-ref: fix DEAD flag contamination of " Tejun Heo
2014-11-22 17:04 ` Shaohua Li
2014-11-22 17:06 ` Tejun Heo
2014-11-23 0:47 ` Shaohua Li [this message]
2014-11-23 17:48 ` Tejun Heo
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