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 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().

-Longman

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