On Thu, Jul 05, 2018 at 09:56:02AM -0700, Paul E. McKenney wrote: > On Thu, Jul 05, 2018 at 05:22:26PM +0100, Will Deacon wrote: > > On Thu, Jul 05, 2018 at 08:44:39AM -0700, Daniel Lustig wrote: > > > On 7/5/2018 8:31 AM, Paul E. McKenney wrote: > > > > On Thu, Jul 05, 2018 at 10:21:36AM -0400, Alan Stern wrote: > > > >> At any rate, it looks like instead of strengthening the relation, I > > > >> should write a patch that removes it entirely. I also will add new, > > > >> stronger relations for use with locking, essentially making spin_lock > > > >> and spin_unlock be RCsc. > > > > > > > > Only in the presence of smp_mb__after_unlock_lock() or > > > > smp_mb__after_spinlock(), correct? Or am I confused about RCsc? > > > > > > > > Thanx, Paul > > > > > > > > > > In terms of naming...is what you're asking for really RCsc? To me, > > > that would imply that even stores in the first critical section would > > > need to be ordered before loads in the second critical section. > > > Meaning that even x86 would need an mfence in either lock() or unlock()? > > > > I think a LOCK operation always implies an atomic RmW, which will give > > full ordering guarantees on x86. I know there have been interesting issues > > involving I/O accesses in the past, but I think that's still out of scope > > for the memory model. > > > > Peter will know. > > Agreed, x86 locked operations imply full fences, so x86 will order the > accesses in consecutive critical sections with respect to an observer > not holding the lock, even stores in earlier critical sections against > loads in later critical sections. We have been discussing tightening > LKMM to make an unlock-lock pair order everything except earlier stores > vs. later loads. (Of course, if everyone holds the lock, they will see > full ordering against both earlier and later critical sections.) > > Or are you pushing for something stronger?
I (and I think Peter) would like something stronger, but we can't have nice things ;) Anyhow, that's not really related to this patch series, so sorry for mis-speaking and thanks to everybody who piled on with corrections! I got a bit arm-centric for a moment. I think Alan got the gist of it, so I'll wait to see what he posts. Will