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

Jason Low
2013-09-30T16:10:50Z

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.