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

From: Tejun Heo
Date: Sun Nov 23 2014 - 12:48:45 EST

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@xxxxxxxxxx>
> Reported-by: Shaohua Li <shli@xxxxxxxxxx>
> Link:
> Fixes: f47ad4578461 ("percpu_ref: decouple switching to percpu mode and reinit")

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


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