Messages in this thread | | | Date | Tue, 12 Nov 2013 09:54:40 -0500 | From | Waiman Long <> | Subject | Re: [PATCH v5 4/4] MCS Lock: Barrier corrections |
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On 11/11/2013 04:17 PM, Tim Chen wrote: >> You could then augment that with [cmp]xchg_{acquire,release} as >> appropriate. >> >>> +/* >>> * In order to acquire the lock, the caller should declare a local node and >>> * pass a reference of the node to this function in addition to the lock. >>> * If the lock has already been acquired, then this will proceed to spin >>> @@ -37,15 +62,19 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node) >>> node->locked = 0; >>> node->next = NULL; >>> >>> - prev = xchg(lock, node); >>> + /* xchg() provides a memory barrier */ >>> + prev = xchg_acquire(lock, node); >>> if (likely(prev == NULL)) { >>> /* Lock acquired */ >>> return; >>> } >>> ACCESS_ONCE(prev->next) = node; >>> - smp_wmb(); >>> - /* Wait until the lock holder passes the lock down */ >>> - while (!ACCESS_ONCE(node->locked)) >>> + /* >>> + * Wait until the lock holder passes the lock down. >>> + * Using smp_load_acquire() provides a memory barrier that >>> + * ensures subsequent operations happen after the lock is acquired. >>> + */ >>> + while (!(smp_load_acquire(&node->locked))) >>> arch_mutex_cpu_relax(); > An alternate implementation is > while (!ACCESS_ONCE(node->locked)) > arch_mutex_cpu_relax(); > smp_load_acquire(&node->locked); > > Leaving the smp_load_acquire at the end to provide appropriate barrier. > Will that be acceptable? > > Tim
I second Tim's opinion. It will be help to have a smp_mb_load_acquire() function that provide a memory barrier with load-acquire semantic. I don't think we need one for store-release as that will not be in a loop.
Peter, what do you think about adding that to your patch?
-Longman
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