On Thu, Aug 20, 2015 at 10:45:05AM +0100, Michael Ellerman wrote: > On Tue, 2015-08-18 at 09:37 +0100, Will Deacon wrote: > > On Tue, Aug 18, 2015 at 02:50:55AM +0100, Michael Ellerman wrote: > > > On Mon, 2015-08-17 at 09:57 +0100, Will Deacon wrote: > > > > On Mon, Aug 17, 2015 at 07:15:01AM +0100, Paul E. McKenney wrote: > > > > > On Mon, Aug 17, 2015 at 02:06:07PM +1000, Michael Ellerman wrote: > > > > > > On Wed, 2015-08-12 at 08:43 -0700, Paul E. McKenney wrote: > > > > > > I thought the end result of this thread was that we didn't *need* > > > > > > to change the > > > > > > powerpc lock semantics? Or did I read it wrong? > > > > > > > > > > > > ie. the docs now say that RELEASE+ACQUIRE is not a full barrier, > > > > > > which is > > > > > > consistent with our current implementation. > > > > > > > > > > That change happened about 1.5 years ago, and I thought that the > > > > > current discussion was about reversing it, based in part on the > > > > > recent powerpc benchmarks of locking primitives with and without the > > > > > sync instruction. But regardless, I clearly cannot remove either the > > > > > smp_mb__after_unlock_lock() or the powerpc definition of it to be > > > > > smp_mb() > > > > > if powerpc unlock/lock is not strengthened. > > > > > > > > Yup. Peter and I would really like to get rid of > > > > smp_mb__after_unlock_lock > > > > entirely, which would mean strengthening the ppc spinlocks. Moving the > > > > barrier primitive into RCU is a good step to prevent more widespread > > > > usage > > > > of the barrier, but we'd really like to go further if the performance > > > > impact > > > > is deemed acceptable (which is what this thread is about). > > > > > > OK, sorry for completely missing the point, too many balls in the air > > > here. > > > > No problem! > > > > > I'll do some benchmarks and see what we come up with. > > > > Thanks, that sounds great. FWIW, there are multiple ways of implementing > > the patch (i.e. whether you strengthen lock or unlock). I had a crack at > > something here, but it's not tested: > > > > http://marc.info/?l=linux-arch&m=143758379023849&w=2 > > Thanks. > > I notice you are not changing PPC_RELEASE_BARRIER, but only the spin unlock > code. But from my reading of the docs we need to make sure any UNLOCK+LOCK is > a > full barrier, not just spin unlock/lock? > > So don't we need to worry about some of the other locks as well? At least > rwlock, and mutex fast path?
Hmm, that's a good question. I notice that you don't do any of the SYNC_IO stuff for any locks other than spinlocks but I don't know whether smp_mb__after_unlock_lock is similarly limited in scope. Paul? 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/
