On Wed, 2015-04-15 at 09:46 +0200, Ingo Molnar wrote: > * Steven Rostedt <rost...@goodmis.org> wrote:
> > You are correct. Now I'm thinking that the WRITE_ONCE() is not needed, > > and just a: > > > > p->mm->numa_scan_seq = READ_ONCE(p->numa_scan_seq) + 1; > > > > Can be done. But I'm still trying to wrap my head around why this is > > needed here. Comments would have been really helpful. We should make > > all READ_ONCE() WRITE_ONCE and obsolete ACCESS_ONCE() have mandatory > > comments just like we do with memory barriers. > > So the original ACCESS_ONCE() barriers were misguided to begin with: I > think they tried to handle races with the scheduler balancing softirq > and tried to avoid having to use atomics for the sequence counter > (which would be overkill), but things like ACCESS_ONCE(x)++ never > guaranteed atomicity (or even coherency) of the update. > > But since in reality this is only statistical sampling code, all these > compiler barriers can be removed I think. Peter, Mel, Rik, do you > agree? So I'll keep the READ_ONCE nested inside WRITE_ONCE for the purpose of this patch since this patch is a conversion from ACCESS_ONCE, but yes, if the original purpose of ACCESS_ONCE was to do an atomic increment, then the ACCESS_ONCE doesn't help with that. -- 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/
