linux-kernel - Re: [PATCH 2/4] locking/rtmutex: Use cmp-cmpxchg

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Date:	Fri, 5 Jun 2015 14:38:16 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Davidlohr Bueso <dave@...olabs.net>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	linux-kernel@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 2/4] locking/rtmutex: Use cmp-cmpxchg

On Tue, 19 May 2015, Davidlohr Bueso wrote:

> Avoid unnecessary cmpxchg calls, all of our other locks
> use it as well.
> 
> Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
> ---
>  kernel/locking/rtmutex.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
> index 74188d8..1d5cc9d 100644
> --- a/kernel/locking/rtmutex.c
> +++ b/kernel/locking/rtmutex.c
> @@ -74,7 +74,9 @@ static void fixup_rt_mutex_waiters(struct rt_mutex *lock)
>   * set up.
>   */
>  #ifndef CONFIG_DEBUG_RT_MUTEXES
> -# define rt_mutex_cmpxchg(l,c,n)	(cmpxchg(&l->owner, c, n) == c)
> +# define rt_mutex_cmpxchg(l,c,n) \
> +	(l->owner == c && cmpxchg(&l->owner, c, n) == c)

Errm. How does that improve stuff like rt_mutex_lock() ?

Thanks,

	tglx
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