On Fri, Sep 20, 2013 at 05:00:19PM +0100, Linus Torvalds wrote: > On Fri, Sep 20, 2013 at 10:45 AM, Will Deacon <will.dea...@arm.com> wrote: > > If we can guarantee that the CODE just messes around with the lockref, those > > barriers probably aren't needed... > > Yes. I've been thyinking about the barrier issue, and as far as I can > see, as long as the lockref code only ever messes with the reference > count, a totally unordered cmpxchg is fine.
The only problem then is the use of cmpxchg64 by the sched_clock code. Whilst most sched_clock() implementations probably have barrier semantics due to I/O access, that's certainly not true everywhere so I don't think the cmpxchg64 there can be relaxed safely. We could add cmpxchg64_relaxed (at the risk of confusing it with the relaxed I/O accessors, which aren't well defined)? That might help Tony with ia64 too. > And at least right now we indeed only ever mess with the reference count. > > I have been idly toying with the concept of using the cmpxchg also for > possibly taking the lock (for the "xyz_or_lock" versions), but every > time I look at it it seems unlikely to help, and it would require > memory ordering and various architecture-dependent issues, so I > suspect it's never going to make much sense. So yes, an unordered > cmpxchg64 should be perfectly fine. Yikes, using cmpxchg for the locking sounds scary. For the contended case, I think spinlocks would be better since they might have back-off and/or fairness logic which we'd lose if we somehow moved exclusively to cmpxchg. > > As for AIM7/re-aim, I'm having a hard time getting repeatable numbers out of > > it to establish a baseline, so it's not proving to be especially helpful. > > That's fine, and yeah, I doubt the t.c improvement really shows > anywhere else (it's kind of extreme), but your numbers are certainly > already sufficient to say "ok, it makes sense even on 32-bit > machines". Great, thanks. Will -- 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/
