linux-kernel - Re: Refactoring mutex spin on owner code

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Date:	Fri, 30 Jan 2015 08:51:40 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Davidlohr Bueso <dave@...olabs.net>
Cc:	Jason Low <jason.low2@...com>, Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Michel Lespinasse <walken@...gle.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Refactoring mutex spin on owner code

On Thu, Jan 29, 2015 at 11:14:40PM -0800, Davidlohr Bueso wrote:
> > +bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
> >  {
> > +	bool ret;
> > +
> >  	rcu_read_lock();
> > -	while (owner_running(lock, owner)) {
> > -		if (need_resched())
> > +	while (true) {
> > +		/* Return success when the lock owner changed */
> > +		if (lock->owner != owner) {
> 
> Shouldn't this be a READ_ONCE(lock->owner)? We're in a loop and need to
> avoid gcc giving us stale data if the owner is updated after a few
> iterations, no?

There's a barrier() in that loop, and cpu_relax() also implies
barrier(). I'm pretty sure that's more than sufficient to make GCC emit
loads.
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