linux-kernel - Re: [PATCH] mutex: Fix mutex_can_spin_on

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Message-ID: <alpine.DEB.2.02.1307192139140.4089@ionos.tec.linutronix.de>
Date:	Fri, 19 Jul 2013 21:41:05 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Waiman Long <waiman.long@...com>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Rik van Riel <riel@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	David Howells <dhowells@...hat.com>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mutex: Fix mutex_can_spin_on_owner

On Fri, 19 Jul 2013, Waiman Long wrote:
> On 07/19/2013 02:31 PM, Peter Zijlstra wrote:
> >   	rcu_read_lock();
> > -	if (lock->owner)
> > -		retval = lock->owner->on_cpu;
> > +	owner = ACCESS_ONCE(lock->owner);
> > +	if (owner)
> > +		retval = owner->on_cpu;
> >   	rcu_read_unlock();
> >   	/*
> >   	 * if lock->owner is not set, the mutex owner may have just
> > acquired
> 

> I am fine with this change. However, the compiler is smart enough to
> not do two memory accesses to the same memory location. So this will
> not change the generated code. Below is the relevant x86 code for
> that section of code:

That's true for your particular compiler, but it's not guaranteed at
all. So it matters even when your compiler generates the same
code. Others might not. There is a world outside of x8664.
 
Thanks,

	tglx
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