Messages in this thread | | | Date | Tue, 11 Feb 2014 10:18:05 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH 5/8] locking, mutex: Cancelable MCS lock for adaptive spinning |
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On Mon, Feb 10, 2014 at 02:04:22PM -0800, Jason Low wrote: > On Mon, 2014-02-10 at 22:32 +0100, Peter Zijlstra wrote: > > 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.
True all; now if only we had a useful benchmark so we could test if it makes a difference or not :-)
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