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

Waiman Long
2013-09-30T15:52:04Z

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