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

[Waiman Long <waiman.long@hp.com>]

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

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>