Messages in this thread | | | Subject | Re: [PATCH 5/8] locking, mutex: Cancelable MCS lock for adaptive spinning | From | Jason Low <> | Date | Mon, 10 Feb 2014 14:04:22 -0800 |
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On Mon, 2014-02-10 at 22:32 +0100, Peter Zijlstra wrote: > On Mon, Feb 10, 2014 at 01:15:59PM -0800, Jason Low wrote: > > On Mon, 2014-02-10 at 20:58 +0100, Peter Zijlstra wrote: > > > +void osq_unlock(struct optimistic_spin_queue **lock) > > > +{ > > > + struct optimistic_spin_queue *node = this_cpu_ptr(&osq_node); > > > + struct optimistic_spin_queue *next; > > > + > > > + /* > > > + * Fast path for the uncontended case. > > > + */ > > > + if (likely(cmpxchg(lock, node, NULL) == node)) > > > + return; > > > > Can we can also add the following code here as I'm noticing next != NULL > > is the much more likely scenario on my box: > > > > next = xchg(&node->next, NULL); > > if (next) { > > ACCESS_ONCE(next->locked) = 1; > > return; > > Is adding that really much faster than the relatively straight path > oqs_wait_next() would walk to bit the same exit? > > The only reason I pulled out the above cmpxchg() is because its the > uncontended fast path, which seems like a special enough case.
So it would avoid 2 extra checks (*lock == node) and (node->next) in the oqs_wait_next() path, which aren't necessary when node->next != NULL.
And I think node->next != NULL can be considered a special enough case after the cmpxchg() fails because in the contended case, we're expecting the node->next to be pointing at something. The only times node->next is NULL after cmpxchg() fails are during a very small race window with the osq_lock(), and when the next node is unqueuing due to need_resched, which is also a very small window.
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