linux-kernel - Re: [PATCH percpu/for-3.18-fixes] percpu-ref: fix DEAD flag contamination of percpu pointer

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Date:	Sat, 22 Nov 2014 16:47:04 -0800
From:	Shaohua Li <shli@...nel.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	linux-kernel@...r.kernel.org, Jens Axboe <axboe@...com>,
	Kent Overstreet <kmo@...erainc.com>
Subject: Re: [PATCH percpu/for-3.18-fixes] percpu-ref: fix DEAD flag
 contamination of percpu pointer

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
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