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

[Jason Low <jason.low2@hp.com>]

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.

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