Messages in this thread | | | Date | Wed, 02 Oct 2013 15:19:29 -0400 | From | Waiman Long <> | Subject | Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines and locking code into its own file |
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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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