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

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Message-ID: <20130625073739.GX28407@twins.programming.kicks-ass.net>
Date:	Tue, 25 Jun 2013 09:37:39 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
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 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.

> 	if (sem_owner)
> 		retval = sem_owner->on_cpu;
> 
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