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

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Date:	Wed, 17 Sep 2014 11:34:37 +0200
From:	Davidlohr Bueso <dbueso@...e.de>
To:	Jason Low <jason.low2@...com>
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>
Subject: Re: [PATCH v3] locking/rwsem: Avoid double checking before try
 acquiring write lock

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!

> Cc: Peter Hurley <peter@...leysoftware.com>
> Cc: Tim Chen <tim.c.chen@...ux.intel.com>

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

> Signed-off-by: Jason Low <jason.low2@...com>

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