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

Waiman Long
2013-10-01T20:01:22Z

On 10/01/2013 12:48 PM, Tim Chen wrote:
> 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

But this is the lock function, a barrier() call should be enough to
prevent the critical section from creeping up there. We certainly need
some kind of memory barrier at the end of the unlock function.

-Longman