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().
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