On Tue, 2015-07-14 at 08:31 +1000, Benjamin Herrenschmidt wrote: > On Mon, 2015-07-13 at 13:15 +0100, Will Deacon wrote: > > smp_mb__after_unlock_lock is used to promote an UNLOCK + LOCK sequence > > into a full memory barrier. > > > > However: > > > > - This ordering guarantee is already provided without the barrier on > > all architectures apart from PowerPC > > > > - The barrier only applies to UNLOCK + LOCK, not general > > RELEASE + ACQUIRE operations > > > > - Locks are generally assumed to offer SC ordering semantics, so > > having this additional barrier is error-prone and complicates the > > callers of LOCK/UNLOCK primitives > > > > - The barrier is not well used outside of RCU and, because it was > > retrofitted into the kernel, it's not clear whether other areas of > > the kernel are incorrectly relying on UNLOCK + LOCK implying a full > > barrier > > > > This patch removes the barrier and instead requires architectures to > > provide full barrier semantics for an UNLOCK + LOCK sequence. > > > > Cc: Benjamin Herrenschmidt <b...@kernel.crashing.org> > > Cc: Paul McKenney <paul...@linux.vnet.ibm.com> > > Cc: Peter Zijlstra <pet...@infradead.org> > > Signed-off-by: Will Deacon <will.dea...@arm.com> > > --- > > > > This didn't go anywhere last time I posted it, but here it is again. > > I'd really appreciate some feedback from the PowerPC guys, especially as > > to whether this change requires them to add an additional barrier in > > arch_spin_unlock and what the cost of that would be. > > We'd have to turn the lwsync in unlock or the isync in lock into a full > barrier. As it is, we *almost* have a full barrier semantic, but not > quite, as in things can get mixed up inside spin_lock between the LL and > the SC (things leaking in past LL and things leaking "out" up before SC > and then getting mixed up in there). > > Michael, at some point you were experimenting a bit with that and tried > to get some perf numbers of the impact that would have, did that > solidify ? Otherwise, I'll have a look when I'm back next week.
I was mainly experimenting with replacing the lwsync in lock with an isync. But I think you're talking about making it a full sync in lock. That was about +7% on p8, +25% on p7 and +88% on p6. We got stuck deciding whether isync was safe to use as a memory barrier, because the wording in the arch is a bit vague. But if we're talking about a full sync then I think there is no question that's OK and we should just do it. cheers -- 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/
