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

From: Jason Low <jason.low2@hp.com>
To: Davidlohr Bueso <dbueso@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>,
	Tim Chen <tim.c.chen@linux.intel.com>,
	Peter Hurley <peter@hurleysoftware.com>,
	linux-kernel@vger.kernel.org,
	Aswin Chandramouleeswaran <aswin@hp.com>,
	Chegu Vinod <chegu_vinod@hp.com>,
	jason.low2@hp.com
Subject: Re: [PATCH v3] locking/rwsem: Avoid double checking before try acquiring write lock
Date: Wed, 17 Sep 2014 09:47:30 -0700	[thread overview]
Message-ID: <1410972450.2447.25.camel@j-VirtualBox> (raw)
In-Reply-To: <1410946476.9389.32.camel@linux-t7sj.site>

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@hurleysoftware.com>
> > Cc: Tim Chen <tim.c.chen@linux.intel.com>
> 
> Acked-by: Davidlohr Bueso <dave@stgolabs.net>

Thanks!