Messages in this thread | | | Date | Mon, 10 Feb 2014 22:32:02 +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 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.
> > + next = osq_wait_next(lock, node, NULL); > > + if (next) > > + ACCESS_ONCE(next->locked) = 1; > > +} >
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