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

[Waiman Long]

On 09/30/2013 12:10 PM, Jason Low wrote:
> On Mon, 2013-09-30 at 11:51 -0400, Waiman Long wrote:
> > On 09/28/2013 12:34 AM, Jason Low wrote:
> > > > Also, below is what the mcs_spin_lock() and mcs_spin_unlock()
> > > > functions would look like after applying the proposed changes.
> > > >
> > > > static noinline
> > > > void mcs_spin_lock(struct mcs_spin_node **lock, struct mcs_spin_node *node)
> > > > {
> > > >           struct mcs_spin_node *prev;
> > > >
> > > >           /* Init node */
> > > >           node->locked = 0;
> > > >           node->next   = NULL;
> > > >
> > > >           prev = xchg(lock, node);
> > > >           if (likely(prev == NULL)) {
> > > >                   /* Lock acquired. No need to set node->locked since it
> > > > won't be used */
> > > >                   return;
> > > >           }
> > > >           ACCESS_ONCE(prev->next) = node;
> > > >           /* Wait until the lock holder passes the lock down */
> > > >           while (!ACCESS_ONCE(node->locked))
> > > >                   arch_mutex_cpu_relax();
> > > >           smp_mb();
> > I wonder if a memory barrier is really needed here.
> If the compiler can reorder the while (!ACCESS_ONCE(node->locked)) check
> so that the check occurs after an instruction in the critical section,
> then the barrier may be necessary.

In that case, just a barrier() call should be enough.

-Longman

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