linux-kernel - Re: [RFC PATCH 3/3] locking/mutex: Optimize mutex trylock slowpath

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Message-ID: <1401913714.13877.11.camel@buesod1.americas.hpqcorp.net>
Date:	Wed, 04 Jun 2014 13:28:34 -0700
From:	Davidlohr Bueso <davidlohr@...com>
To:	Jason Low <jason.low2@...com>
Cc:	mingo@...nel.org, peterz@...radead.org, tglx@...utronix.de,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	paulmck@...ux.vnet.ibm.com, tim.c.chen@...ux.intel.com,
	peter@...leysoftware.com, riel@...hat.com, hpa@...or.com,
	walken@...gle.com, Waiman.Long@...com, aswin@...com,
	scott.norton@...com, chegu_vinod@...com
Subject: Re: [RFC PATCH 3/3] locking/mutex: Optimize mutex trylock slowpath

On Wed, 2014-06-04 at 12:08 -0700, Jason Low wrote:
> In __mutex_trylock_slowpath(), we acquire the wait_lock spinlock,
> xchg() lock->count with -1, then set lock->count back to 0 if there
> are no waiters, and return true if the prev lock count was 1.
> 
> However, if we the mutex is already locked, then there may not be
             ^^ leave that out.

> much point in attempting the above operations. 

Isn't this redundant? I mean, if we enter the slowpath its because
__mutex_fastpath_trylock() already failed so we already know that the
lock is taken.

What kind of testing has this change been put through? Any advantages?
(ie: how many cycles are we saving here?), the trylock mechanism is
already pretty darn fast.

Thanks,
Davidlohr

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