linux-kernel - Re: [PATCH v3] locking/rwsem: Avoid double checking before try acquiring write lock

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Date:	Wed, 17 Sep 2014 09:47:30 -0700
From:	Jason Low <jason.low2@...com>
To:	Davidlohr Bueso <dbueso@...e.de>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Peter Hurley <peter@...leysoftware.com>,
	linux-kernel@...r.kernel.org,
	Aswin Chandramouleeswaran <aswin@...com>,
	Chegu Vinod <chegu_vinod@...com>, jason.low2@...com
Subject: Re: [PATCH v3] locking/rwsem: Avoid double checking before try
 acquiring write lock

On Wed, 2014-09-17 at 11:34 +0200, Davidlohr Bueso wrote:
> On Tue, 2014-09-16 at 17:16 -0700, Jason Low wrote:
> > Commit 9b0fc9c09f1b checks for if there are known active lockers
> > in order to avoid write trylocking using expensive cmpxchg() when
> > it likely wouldn't get the lock.
> 
> Ah, I remember you had this one in your queue for some time :)
> 
> > However, a subsequent patch was added such that we directly
> > check for sem->count == RWSEM_WAITING_BIAS right before trying
> > that cmpxchg(). Thus, commit 9b0fc9c09f1b now just adds overhead.
> > This patch modifies it so that we only do a check for if
> > count == RWSEM_WAITING_BIAS.
> > 
> > Also, add a comment on why we do an "extra check" of count
> > before the cmpxchg().
> 
> heh, so people don't try to remove the "redundant" check!

Right, I've already seen at least 1 such patch  :)

> > Cc: Peter Hurley <peter@...leysoftware.com>
> > Cc: Tim Chen <tim.c.chen@...ux.intel.com>
> 
> Acked-by: Davidlohr Bueso <dave@...olabs.net>

Thanks!

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