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@lists.linux-foundation.org
https://lists.linux-foundation.org/mailman/listinfo/virtualization
