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Subject: Re: [PATCH v5 1/6] rwsem: check the lock before cpmxchg in
 down_write_trylock
From: Tim Chen <tim.c.chen@linux.intel.com>
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Date: Tue, 24 Sep 2013 16:30:19 -0700
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To: Jason Low <jason.low2@hp.com>
Cc: Ingo Molnar <mingo@elte.hu>, Andrew Morton <akpm@linux-foundation.org>, Andrea Arcangeli <aarcange@redhat.com>, Alex Shi <alex.shi@linaro.org>, Andi Kleen <andi@firstfloor.org>, Michel Lespinasse <walken@google.com>, Davidlohr Bueso <davidlohr.bueso@hp.com>, Matthew R Wilcox <matthew.r.wilcox@intel.com>, Dave Hansen <dave.hansen@intel.com>, Peter Zijlstra <a.p.zijlstra@chello.nl>, Rik van Riel <riel@redhat.com>, Peter Hurley <peter@hurleysoftware.com>, linux-kernel@vger.kernel.org, linux-mm <linux-mm@kvack.org>

On Tue, 2013-09-24 at 16:22 -0700, Jason Low wrote:
> Should we do something similar with __down_read_trylock, such as
> the following?
> 
> 
> Signed-off-by: Jason Low <jason.low2@hp.com>
> ---
>  include/asm-generic/rwsem.h |    3 +++
>  1 files changed, 3 insertions(+), 0 deletions(-)
> 
> diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h
> index bb1e2cd..47990dc 100644
> --- a/include/asm-generic/rwsem.h
> +++ b/include/asm-generic/rwsem.h
> @@ -42,6 +42,9 @@ static inline int __down_read_trylock(struct
> rw_semaphore *sem)
>         long tmp;
> 
>         while ((tmp = sem->count) >= 0) {
> +               if (sem->count != tmp)
> +                       continue;
> +

Considering that tmp has just been assigned the value of sem->count, the
added if check failure is unlikely and probably not needed.  We should
proceed to cmpxchg below.

>                 if (tmp == cmpxchg(&sem->count, tmp,
>                                    tmp + RWSEM_ACTIVE_READ_BIAS)) {
>                         return 1;

Tim

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