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

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Date:	Sun, 23 Nov 2014 12:48:38 -0500
From:	Tejun Heo <tj@...nel.org>
To:	Shaohua Li <shli@...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 09:22:42AM -0500, Tejun Heo wrote:
> While decoupling ATOMIC and DEAD flags, f47ad4578461 ("percpu_ref:
> decouple switching to percpu mode and reinit") updated
> __ref_is_percpu() so that it only tests ATOMIC flag to determine
> whether the ref is in percpu mode or not; however, while DEAD implies
> ATOMIC, the two flags are set separately during percpu_ref_kill() and
> if __ref_is_percpu() races percpu_ref_kill(), it may see DEAD w/o
> ATOMIC.  Because __ref_is_percpu() returns @ref->percpu_count_ptr
> value verbatim as the percpu pointer after testing ATOMIC, the pointer
> may now be contaminated with the DEAD flag.
> 
> This can be fixed by clearing the flag bits before returning the
> pointer which was the fix proposed by Shaohua; however, as DEAD
> implies ATOMIC, we can just test for both flags at once and avoid the
> explicit masking.
> 
> Update __ref_is_percpu() so that it tests that both ATOMIC and DEAD
> are clear before returning @ref->percpu_count_ptr as the percpu
> pointer.
> 
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Reported-by: Shaohua Li <shli@...nel.org>
> Link: http://lkml.kernel.org/r/995deb699f5b873c45d667df4add3b06f73c2c25.1416638887.git.shli@kernel.org
> Fixes: f47ad4578461 ("percpu_ref: decouple switching to percpu mode and reinit")

Applied to percpu/for-3.18-fixes and pushed out to Linus.

Thanks.

-- 
tejun
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