Messages in this thread | | | Date | Wed, 2 Oct 2013 12:30:00 -0700 | Subject | Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines and locking code into its own file | From | Jason Low <> |
| |
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.
| |