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Date: Tue, 25 Jun 2013 09:37:39 +0200
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 2/2] rwsem: do optimistic spinning for writer lock
 acquisition
Message-ID: <20130625073739.GX28407@twins.programming.kicks-ass.net>
References: <cover.1371855277.git.tim.c.chen@linux.intel.com>
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To: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Peter Hurley <peter@hurleysoftware.com>, Davidlohr Bueso <davidlohr.bueso@hp.com>, Alex Shi <alex.shi@intel.com>, Michel Lespinasse <walken@google.com>, Ingo Molnar <mingo@elte.hu>, Andrew Morton <akpm@linux-foundation.org>, Andrea Arcangeli <aarcange@redhat.com>, Andi Kleen <andi@firstfloor.org>, Matthew R Wilcox <matthew.r.wilcox@intel.com>, Dave Hansen <dave.hansen@intel.com>, Rik van Riel <riel@redhat.com>, linux-kernel@vger.kernel.org, linux-mm <linux-mm@kvack.org>

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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