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

Waiman Long
2013-10-02T19:37:43Z

On 10/02/2013 03:30 PM, Jason Low wrote:
> On Wed, Oct 2, 2013 at 12:19 PM, Waiman Long<waiman.long@hp.com> wrote:
>> On 09/26/2013 06:42 PM, Jason Low wrote:
>>> On Thu, 2013-09-26 at 14:41 -0700, Tim Chen wrote:
>>>> Okay, that would makes sense for consistency because we always
>>>> first set node->lock = 0 at the top of the function.
>>>>
>>>> If we prefer to optimize this a bit though, perhaps we can
>>>> first move the node->lock = 0 so that it gets executed after the
>>>> "if (likely(prev == NULL)) {}" code block and then delete
>>>> "node->lock = 1" inside the code block.
>>>>
>>>> static noinline
>>>> void mcs_spin_lock(struct mcs_spin_node **lock, struct mcs_spin_node
>>>> *node)
>>>> {
>>>> struct mcs_spin_node *prev;
>>>>
>>>> /* Init node */
>>>> node->next = NULL;
>>>>
>>>> prev = xchg(lock, node);
>>>> if (likely(prev == NULL)) {
>>>> /* Lock acquired */
>>>> return;
>>>> }
>>>> node->locked = 0;
>>
>> You can remove the locked flag setting statement inside if (prev == NULL),
>> but you can't clear the locked flag after xchg(). In the interval between
>> xchg() and locked=0, the previous lock owner may come in and set the flag.
>> Now if your clear it, the thread will loop forever. You have to clear it
>> before xchg().
> Yes, in my most recent version, I left locked = 0 in its original
> place so that the xchg() can act as a barrier for it.
>
> The other option would have been to put another barrier after locked =
> 0. I went with leaving locked = 0 in its original place so that we
> don't need that extra barrier.

I don't think putting another barrier after locked=0 will work.
Chronologically, the flag must be cleared before the node address is
saved in the lock field. There is no way to guarantee that except by
putting the locked=0 before xchg().

-Longman