From: Davidlohr Bueso <dbueso@suse.de>
To: Jason Low <jason.low2@hp.com>
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>
Subject: Re: [PATCH v3] locking/rwsem: Avoid double checking before try acquiring write lock
Date: Wed, 17 Sep 2014 11:34:37 +0200 [thread overview]
Message-ID: <1410946476.9389.32.camel@linux-t7sj.site> (raw)
In-Reply-To: <1410913017.2447.22.camel@j-VirtualBox>
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@hurleysoftware.com>
> Cc: Tim Chen <tim.c.chen@linux.intel.com>
Acked-by: Davidlohr Bueso <dave@stgolabs.net>
> Signed-off-by: Jason Low <jason.low2@hp.com>
next prev parent reply other threads:[~2014-09-17 9:36 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-09-17 0:16 Jason Low
2014-09-17 9:34 ` Davidlohr Bueso [this message]
2014-09-17 16:47 ` Jason Low
2014-10-03 5:29 ` [tip:locking/core] " tip-bot for Jason Low
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