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

[Waiman Long]

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.

> > }
> >
> > static void mcs_spin_unlock(struct mcs_spin_node **lock, struct
> > mcs_spin_node *node)
> > {
> >          struct mcs_spin_node *next = ACCESS_ONCE(node->next);
> >
> >          if (likely(!next)) {
> >                  /*
> >                   * Release the lock by setting it to NULL
> >                   */
> >                  if (cmpxchg(lock, node, NULL) == node)
> >                          return;
> >                  /* Wait until the next pointer is set */
> >                  while (!(next = ACCESS_ONCE(node->next)))
> >                          arch_mutex_cpu_relax();
> >          }
> >          smp_wmb();
> >          ACCESS_ONCE(next->locked) = 1;
> > }

Instead, I think what we need may be:

if (likely(!next)) {
    ....
} else
    smp_mb();
ACCESS_ONCE(next->locked) = 1;

That will ensure a memory barrier in the unlock path.

Regards,
Longman
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