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

[Tejun Heo]

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.

Thanks.

-- 
tejun
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