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

[Shaohua Li]

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: http://lkml.kernel.org/r/995deb699f5b873c45d667df4add3b06f73c2c25.1416638887.git.shli@xxxxxxxxxx
> Fixes: f47ad4578461 ("percpu_ref: decouple switching to percpu mode and reinit")
> ---
> Hello, Shaohua.
> 
> That was a nasty one.  I think this fix is slightly better.  Can you
> please confirm that this fixes the issues you're seeing too?
> 
> Thanks.
> 
>  include/linux/percpu-refcount.h |    8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h
> index d5c89e0..51ce60c 100644
> --- a/include/linux/percpu-refcount.h
> +++ b/include/linux/percpu-refcount.h
> @@ -133,7 +133,13 @@ static inline bool __ref_is_percpu(struct percpu_ref *ref,
>  	/* paired with smp_store_release() in percpu_ref_reinit() */
>  	smp_read_barrier_depends();
>  
> -	if (unlikely(percpu_ptr & __PERCPU_REF_ATOMIC))
> +	/*
> +	 * 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.

Thanks,
Shaohua
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