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

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Date:	Wed, 04 Jun 2014 12:44:36 -0700
From:	Davidlohr Bueso <davidlohr@...com>
To:	Andev <debiandev@...il.com>
Cc:	paulmck@...ux.vnet.ibm.com, Tim Chen <tim.c.chen@...ux.intel.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>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] rwsem: Support optimistic spinning

On Wed, 2014-06-04 at 13:57 -0400, Andev wrote:
> Hi,
> 
> On Tue, Apr 29, 2014 at 11:11 AM, Paul E. McKenney
> <paulmck@...ux.vnet.ibm.com> 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?  If so, I am OK with it as long as there
> > is a comment (which Davidlohr suggested later in this thread).
> >
> >                                                         Thanx, Paul
> >
> 
> The latest code seems to be missing this comment. Could you please add this?

The comment is there when we declare ->owner in struct rw_semaphore.


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