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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