Il 20/08/2014 18:01, Radim Krčmář ha scritto: > 2014-08-20 17:34+0200, Paolo Bonzini: >> Il 20/08/2014 17:31, Radim Krčmář ha scritto: >>> Btw. without extra code, we are still going to overflow on races when >>> changing PW_grow, should they be covered as well? >> >> You mean because there is no spinlock or similar protecting the changes? >> I guess you could use a seqlock. > > Yes, for example between a modification of ple_window > new = min(old, PW_actual_max) * PW_grow > which gets compiled into something like this: > 1) tmp = min(old, PW_actual_max) > 2) new = tmp * PW_grow > and a write to increase PW_grow > 3) PW_actual_max = min(PW_max / new_PW_grow, PW_actual_max) > 4) PW_grow = new_PW_grow > 5) PW_actual_max = PW_max / new_PW_grow > > 3 and 4 can exectute between 1 and 2, which could overflow. > > I don't think they are important enough to warrant a significant > performance hit of locking.
A seqlock just costs two memory accesses to the same (shared) cache line as the PW data, and a non-taken branch. I don't like code that is unsafe by design... Paolo > Or even more checks that would prevent it in a lockless way. > > (I'd just see that the result is set to something legal and also drop > line 3, because it does not help things that much.) > -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
