Re: [PATCH v2 3/4] MCS Lock: Barrier corrections

[Paul E. McKenney]

On Tue, Nov 05, 2013 at 10:18:03PM +0100, Peter Zijlstra wrote:
> On Tue, Nov 05, 2013 at 11:21:57AM -0800, Tim Chen wrote:
> > On Tue, 2013-11-05 at 18:37 +0000, Will Deacon wrote:
> > > On Tue, Nov 05, 2013 at 05:42:36PM +0000, Tim Chen wrote:
> > > > This patch corrects the way memory barriers are used in the MCS lock
> > > > and removes ones that are not needed. Also add comments on all barriers.
> > > 
> > > Hmm, I see that you're fixing up the barriers, but I still don't completely
> > > understand how what you have is correct. Hopefully you can help me out :)
> > > 
> > > > Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > > > Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> > > > Signed-off-by: Jason Low <jason.low2@xxxxxx>
> > > > ---
> > > >  include/linux/mcs_spinlock.h |   13 +++++++++++--
> > > >  1 files changed, 11 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/include/linux/mcs_spinlock.h b/include/linux/mcs_spinlock.h
> > > > index 96f14299..93d445d 100644
> > > > --- a/include/linux/mcs_spinlock.h
> > > > +++ b/include/linux/mcs_spinlock.h
> > > > @@ -36,16 +36,19 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
> > > >  	node->locked = 0;
> > > >  	node->next   = NULL;
> > > >  
> > > > +	/* xchg() provides a memory barrier */
> > > >  	prev = xchg(lock, node);
> > > >  	if (likely(prev == NULL)) {
> > > >  		/* Lock acquired */
> > > >  		return;
> > > >  	}
> > > >  	ACCESS_ONCE(prev->next) = node;
> > > > -	smp_wmb();
> > > >  	/* Wait until the lock holder passes the lock down */
> > > >  	while (!ACCESS_ONCE(node->locked))
> > > >  		arch_mutex_cpu_relax();
> > > > +
> > > > +	/* Make sure subsequent operations happen after the lock is acquired */
> > > > +	smp_rmb();
> > > 
> > > Ok, so this is an smp_rmb() because we assume that stores aren't speculated,
> > > right? (i.e. the control dependency above is enough for stores to be ordered
> > > with respect to taking the lock)...
> 
> PaulMck completely confused me a few days ago with control dependencies
> etc.. Pretty much saying that C/C++ doesn't do those.

I remember that there was a subtlety here, but don't remember what it was...

And while I do remember reviewing this code, I don't find any evidence
that I gave my "Reviewed-by".  Tim/Jason, if I fat-fingered this, please
forward that email back to me.

							Thanx, Paul

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