On Wed, Apr 01, 2015 at 07:12:23PM +0200, Peter Zijlstra wrote: > On Wed, Apr 01, 2015 at 12:20:30PM -0400, Waiman Long wrote: > > After more careful reading, I think the assumption that the presence of an > > unused bucket means there is no match is not true. Consider the scenario: > > > > 1. cpu 0 puts lock1 into hb[0] > > 2. cpu 1 puts lock2 into hb[1] > > 3. cpu 2 clears hb[0] > > 4. cpu 3 looks for lock2 and doesn't find it > > Hmm, yes. The only way I can see that being true is if we assume entries > are never taken out again. > > The wikipedia page could use some clarification here, this is not clear. > > > At this point, I am thinking using back your previous idea of passing the > > queue head information down the queue. > > Having to scan the entire array for a lookup sure sucks, but the wait > loops involved in the other idea can get us in the exact predicament we > were trying to get out, because their forward progress depends on other > CPUs. > > Hohumm.. time to think more I think ;-) So bear with me, I've not really pondered this well so it could be full of holes (again). After the cmpxchg(&l->locked, _Q_LOCKED_VAL, _Q_SLOW_VAL) succeeds the spin_unlock() must do the hash lookup, right? We can make the lookup unhash. If the cmpxchg() fails the unlock will not do the lookup and we must unhash. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
