linux-kernel - Re: [RFC][PATCH] mutex: Optimize mutex

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Message-ID: <20160602142619.nha6gcs5x4nmwyu3@linux-uzut.site>
Date:	Thu, 2 Jun 2016 07:26:20 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>, Waiman Long <Waiman.Long@....com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	viro@...IV.linux.org.uk, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] mutex: Optimize mutex_trylock() fast-path

On Wed, 01 Jun 2016, Peter Zijlstra wrote:

>
>A while back Viro posted a number of 'interesting' mutex_is_locked()
>users on IRC, one of those was RCU.
>
>RCU seems to use mutex_is_locked() to avoid doing mutex_trylock(), the
>regular load before modify pattern.
>
>While the use isn't wrong per se, its curious in that its needed at all,
>mutex_trylock() should be good enough on its own to avoid the pointless
>cacheline bounces.

Yeah, and we use ccas just about everywhere else for mutexes.

>
>So fix those and remove the mutex_is_locked() (ab)use from RCU.
>
>Reported-by: Al Viro <viro@...IV.linux.org.uk>
>Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>

Acked-by: Davidlohr Bueso <dave@...olabs.net>