linux-kernel - Re: [PATCH v2] rwsem: Support optimistic spinning

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Date:	Tue, 29 Apr 2014 09:00:05 -0700
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Davidlohr Bueso <davidlohr@...com>, Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Alex Shi <alex.shi@...aro.org>,
	Andi Kleen <andi@...stfloor.org>,
	Michel Lespinasse <walken@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Aswin Chandramouleeswaran <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] rwsem: Support optimistic spinning

On Tue, 2014-04-29 at 08:11 -0700, Paul E. McKenney wrote:
> On Mon, Apr 28, 2014 at 05:50:49PM -0700, Tim Chen wrote:
> > On Mon, 2014-04-28 at 16:10 -0700, Paul E. McKenney wrote:
> > 
> > > > +#ifdef CONFIG_SMP
> > > > +static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
> > > > +{
> > > > +	int retval;
> > > > +	struct task_struct *owner;
> > > > +
> > > > +	rcu_read_lock();
> > > > +	owner = ACCESS_ONCE(sem->owner);
> > > 
> > > OK, I'll bite...
> > > 
> > > Why ACCESS_ONCE() instead of rcu_dereference()?
> > 
> > We're using it as a speculative check on the sem->owner to see
> > if the owner is running on the cpu.  The rcu_read_lock
> > is used for ensuring that the owner->on_cpu memory is
> > still valid.
> 
> OK, so if we read complete garbage, all that happens is that we
> lose a bit of performance?  

Correct.

> If so, I am OK with it as long as there
> is a comment (which Davidlohr suggested later in this thread).
> 
Yes, we should add some comments to clarify things.

Thanks.

Tim


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