linux-kernel - Re: [PATCH 2/2] rwsem: do optimistic spinning for writer lock acquisition

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Date:	Tue, 25 Jun 2013 09:00:06 -0700
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Peter Hurley <peter@...leysoftware.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Alex Shi <alex.shi@...el.com>,
	Michel Lespinasse <walken@...gle.com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Matthew R Wilcox <matthew.r.wilcox@...el.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH 2/2] rwsem: do optimistic spinning for writer lock
 acquisition

On Tue, 2013-06-25 at 09:37 +0200, Peter Zijlstra wrote:
> On Mon, Jun 24, 2013 at 01:17:45PM -0700, Tim Chen wrote:
> > On second thought, I agree with you.  I should change this to
> > something like
> > 
> > 	int retval = true;
> > 	task_struct *sem_owner;
> > 
> > 	/* Spin only if active writer running */
> > 	if (!sem->owner)
> > 		return false;
> > 
> > 	rcu_read_lock();
> > 	sem_owner = sem->owner;
> 
> That should be: sem_owner = ACCESS_ONCE(sem->owner); to make sure the
> compiler doesn't try and be clever and rereads.

Thanks.  Will incorporate this in next version of the patch.

Tim


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