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

[Tim Chen]

On Mon, 2013-09-30 at 12:36 -0400, Waiman Long wrote:
> 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.

The cpu could still be executing out of order load instruction from the
critical section before checking node->locked?  Probably smp_mb() is
still needed.

Tim


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