Messages in this thread | | | Date | Tue, 01 Oct 2013 16:01:05 -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 10/01/2013 12:48 PM, Tim Chen wrote: > On Mon, 2013-09-30 at 12:36 -0400, Waiman Long wrote: >> On 09/30/2013 12:10 PM, Jason Low wrote: >>> On Mon, 2013-09-30 at 11:51 -0400, Waiman Long wrote: >>>> On 09/28/2013 12:34 AM, Jason Low wrote: >>>>>> Also, below is what the mcs_spin_lock() and mcs_spin_unlock() >>>>>> functions would look like after applying the proposed changes. >>>>>> >>>>>> static noinline >>>>>> void mcs_spin_lock(struct mcs_spin_node **lock, struct mcs_spin_node *node) >>>>>> { >>>>>> struct mcs_spin_node *prev; >>>>>> >>>>>> /* Init node */ >>>>>> node->locked = 0; >>>>>> node->next = NULL; >>>>>> >>>>>> prev = xchg(lock, node); >>>>>> if (likely(prev == NULL)) { >>>>>> /* Lock acquired. No need to set node->locked since it >>>>>> won't be used */ >>>>>> return; >>>>>> } >>>>>> ACCESS_ONCE(prev->next) = node; >>>>>> /* Wait until the lock holder passes the lock down */ >>>>>> while (!ACCESS_ONCE(node->locked)) >>>>>> arch_mutex_cpu_relax(); >>>>>> smp_mb(); >>>> I wonder if a memory barrier is really needed here. >>> If the compiler can reorder the while (!ACCESS_ONCE(node->locked)) check >>> so that the check occurs after an instruction in the critical section, >>> then the barrier may be necessary. >>> >> In that case, just a barrier() call should be enough. > The cpu could still be executing out of order load instruction from the > critical section before checking node->locked? Probably smp_mb() is > still needed. > > Tim
But this is the lock function, a barrier() call should be enough to prevent the critical section from creeping up there. We certainly need some kind of memory barrier at the end of the unlock function.
-Longman
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