On 02/10, Jeremy Fitzhardinge wrote: > > On 02/10/2015 05:26 AM, Oleg Nesterov wrote: > > On 02/10, Raghavendra K T wrote: > >> Unfortunately xadd could result in head overflow as tail is high. > >> > >> The other option was repeated cmpxchg which is bad I believe. > >> Any suggestions? > > Stupid question... what if we simply move SLOWPATH from .tail to .head? > > In this case arch_spin_unlock() could do xadd(tickets.head) and check > > the result > > Well, right now, "tail" is manipulated by locked instructions by CPUs > who are contending for the ticketlock, but head can be manipulated > unlocked by the CPU which currently owns the ticketlock. If SLOWPATH > moved into head, then non-owner CPUs would be touching head, requiring > everyone to use locked instructions on it. > > That's the theory, but I don't see much (any?) code which depends on that. > > Ideally we could find a way so that pv ticketlocks could use a plain > unlocked add for the unlock like the non-pv case, but I just don't see a > way to do it.
I agree, and I have to admit I am not sure I fully understand why unlock uses the locked add. Except we need a barrier to avoid the race with the enter_slowpath() users, of course. Perhaps this is the only reason? Anyway, I suggested this to avoid the overflow if we use xadd(), and I guess we need the locked insn anyway if we want to eliminate the unsafe read-after-unlock... > > BTW. If we move "clear slowpath" into "lock" path, then probably trylock > > should be changed too? Something like below, we just need to clear SLOWPATH > > before cmpxchg. > > How important / widely used is trylock these days? I am not saying this is that important. Just this looks more consistent imo and we can do this for free. Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
