On Mon, 2010-11-15 at 12:03 -0800, H. Peter Anvin wrote: > On 11/15/2010 12:00 PM, Jeremy Fitzhardinge wrote: > > > > Another approach I discussed with PeterZ and Mathieu is to steal the LSB > > of the ticket counters (halving the max CPU count) to use as a "there is > > someone in slowpath waiting on this lock". But I haven't spent the time > > to work out an algorithm to maintain that flag (or flags, since there > > are bits available) in a correct and efficient way. > > > > Definitely worth pondering. Right, so the idea was to make the ticket increment 2, which would leave the LSB of both the head and tail available. I think that if one were to set both (using a cmpxchg), the ticket fast-path wouldn't need any changes since head==tail is still the correct condition for acquisition. Then the unlock needs an added conditional: if (tail & 1) unlock_slowpath() _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/virtualization
