linux-kernel - Re: [RFC PATCH 1/1] remove redundant compare, cmpxchg already does it

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Message-ID: <CAJUuVQ4VioKuVuCTXr3BcfRpYs+9+5bx9m2WLO7Swsnni0tUAw@mail.gmail.com>
Date:	Wed, 4 Jun 2014 16:56:50 -0400
From:	Andev <debiandev@...il.com>
To:	Pranith Kumar <pranith@...ech.edu>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 1/1] remove redundant compare, cmpxchg already does it

On Wed, Jun 4, 2014 at 4:38 PM, Pranith Kumar <pranith@...ech.edu> wrote:
> remove a redundant comparision
>
> Signed-off-by: Pranith Kumar <bobby.prani@...il.com>
> ---
>  kernel/locking/rwsem-xadd.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
> index 1f99664b..6f8bd3c 100644
> --- a/kernel/locking/rwsem-xadd.c
> +++ b/kernel/locking/rwsem-xadd.c
> @@ -249,8 +249,7 @@ static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem)
>  {
>      if (!(count & RWSEM_ACTIVE_MASK)) {
>          /* try acquiring the write lock */
> -        if (sem->count == RWSEM_WAITING_BIAS &&
> -            cmpxchg(&sem->count, RWSEM_WAITING_BIAS,
> +        if (cmpxchg(&sem->count, RWSEM_WAITING_BIAS,
>                  RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_WAITING_BIAS) {

This was mainly done to avoid the cost of a cmpxchg in case where they
are not equal. Not sure if it really makes a difference though.

-- 
Andev
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