On Thu, Dec 19, 2013 at 06:07:41PM +0100, Ingo Molnar wrote: > > * H. Peter Anvin <hpa@xxxxxxxxx> wrote: > > > On 12/19/2013 08:21 AM, Peter Zijlstra wrote: > > > > > > What's that mb for? > > > > > > > It already exists in mwait_idle_with_hints(); I just moved it into > > this common function. It is a bit odd, I have to admit; it seems > > like it should be *before* the monitor (and possibly we should have > > one after the CLFLUSH as well?) > > Yes, I think we need a barrier before the CLFLUSH, because according > to my reading of the Intel documentation CLFLUSH has no implicit > ordering so it might get reordered with the store to ->flags in > current_set_polling_and_test(), which might result in spurious wakeup > problems again. No it cannot; since current_set_polling_and_test() already has a barrier to prevent that. Also, the location patched by hpa doesn't actually call that at all. That said, I would find it very strange indeed if a CLFLUSH doesn't also flush the store buffer. > (And CLFLUSH is a store in a sense, so special in that the regular > ordering for stores does not apply.) > > Likewise, having a barrier before the MONITOR looks sensible as well. > Having it _after_ monitor looks weird and is probably wrong. [It might > have been the effects of someone seeing the spurious wakeup problems > with realizing the true source, or so.] I again have to disagree, one would expect monitor to flush all that is required to start the monitor -- and it actually does so. As is testified by this extra CLFLUSH being called a bug workaround. -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html