linux-kernel - Re: [PATCH v5 1/6] rwsem: check the lock before cpmxchg in down_write

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

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@...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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