Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines andlocking code into its own file

[Tim Chen]

On Fri, 2013-09-27 at 19:19 -0700, Paul E. McKenney wrote:
> On Fri, Sep 27, 2013 at 04:54:06PM -0700, Jason Low wrote:
> > On Fri, Sep 27, 2013 at 4:01 PM, Paul E. McKenney
> > <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> > > Yep.  The previous lock holder's smp_wmb() won't keep either the compiler
> > > or the CPU from reordering things for the new lock holder.  They could for
> > > example reorder the critical section to precede the node->locked check,
> > > which would be very bad.
> > 
> > Paul, Tim, Longman,
> > 
> > How would you like the proposed changes below?
> 
> Could you point me at what this applies to?  I can find flaws looking
> at random pieces, given a little luck, but at some point I need to look
> at the whole thing.  ;-)
> 
> 							Thanx, Paul

Jason's patch is on top of the following patchset:

https://lkml.org/lkml/2013/9/26/674

Thanks.

Tim

> 
> > ---
> > Subject: [PATCH] MCS: optimizations and barrier corrections
> > 
> > Delete the node->locked = 1 assignment if the lock is free as it won't be used.
> > 
> > Delete the smp_wmb() in mcs_spin_lock() and add a full memory barrier at the
> > end of the mcs_spin_lock() function. As Paul McKenney suggested, "you do need a
> > full memory barrier here in order to ensure that you see the effects of the
> > previous lock holder's critical section." And in the mcs_spin_unlock(), move the
> > memory barrier so that it is before the "ACCESS_ONCE(next->locked) = 1;".
> > 
> > Signed-off-by: Jason Low <jason.low2@xxxxxx>
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> > ---
> >  include/linux/mcslock.h |    7 +++----
> >  1 files changed, 3 insertions(+), 4 deletions(-)
> > 
> > diff --git a/include/linux/mcslock.h b/include/linux/mcslock.h
> > index 20fd3f0..edd57d2 100644
> > --- a/include/linux/mcslock.h
> > +++ b/include/linux/mcslock.h
> > @@ -26,15 +26,14 @@ void mcs_spin_lock(struct mcs_spin_node **lock,
> > struct mcs_spin_node *node)
> > 
> >         prev = xchg(lock, node);
> >         if (likely(prev == NULL)) {
> > -               /* Lock acquired */
> > -               node->locked = 1;
> > +               /* Lock acquired. No need to set node->locked since it
> > won't be used */
> >                 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();
> > +       smp_mb();
> >  }
> > 
> >  static void mcs_spin_unlock(struct mcs_spin_node **lock, struct
> > mcs_spin_node *node)
> > @@ -51,8 +50,8 @@ static void mcs_spin_unlock(struct mcs_spin_node
> > **lock, struct mcs_spin_node *n
> >                 while (!(next = ACCESS_ONCE(node->next)))
> >                         arch_mutex_cpu_relax();
> >         }
> > -       ACCESS_ONCE(next->locked) = 1;
> >         smp_wmb();
> > +       ACCESS_ONCE(next->locked) = 1;
> >  }
> > 
> >  #endif
> > -- 
> > 1.7.1
> > 
> 
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