linux-kernel - Re: [PATCH v2] locking/rwsem: Avoid double checking before try acquiring write lock

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Date:	Tue, 16 Sep 2014 15:01:07 -0700
From:	Jason Low <jason.low2@...com>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Davidlohr Bueso <dbueso@...e.de>, linux-kernel@...r.kernel.org,
	Aswin Chandramouleeswaran <aswin@...com>,
	Chegu Vinod <chegu_vinod@...com>, jason.low2@...com
Subject: Re: [PATCH v2] locking/rwsem: Avoid double checking before try
 acquiring write lock

On Tue, 2014-09-16 at 16:08 -0400, Peter Hurley wrote:
> Hi Jason,
> 
> On 09/16/2014 03:01 PM, 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.
> > 
> > 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 extra overhead. This patch
> > deletes it.
> 
> It would be better to just not reload sem->count, and check the parameter
> count == RWSEM_WAITING_BIAS instead. The count parameter is a very recent
> load of sem->count (one of which is the latest exclusive read from an
> atomic operation), so likely to be just as accurate as a reload of
> sem->count without causing more cache line contention.

Hi Peter and Tim,

Yes, I also agree. I will send out a new patch with this update.

Thanks,
Jason

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