Messages in this thread | | | Date | Wed, 02 Oct 2013 15:37:26 -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 10/02/2013 03:30 PM, Jason Low wrote: > 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.
I don't think putting another barrier after locked=0 will work. Chronologically, the flag must be cleared before the node address is saved in the lock field. There is no way to guarantee that except by putting the locked=0 before xchg().
-Longman
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