On Tue, Jul 14, 2015 at 11:04:29AM +0100, Will Deacon wrote: > On Tue, Jul 14, 2015 at 12:04:06AM +0100, Paul E. McKenney wrote: > > On Tue, Jul 14, 2015 at 12:23:46AM +0200, Peter Zijlstra wrote: > > > If we look at the inside of the critical section again -- similar > > > argument as before: > > > > > > *A = a > > > smp_mb() > > > store M > > > load N > > > smp_mb() > > > *B = b > > > > > > A and B are fully ordered, and in this case even transitivity is > > > provided. > > > > > > I'm stating that the order of M and N don't matter, only the > > > load/stores that are inside the acquire/release are constrained. > > > > No argument here. > > > > > IOW, I think smp_mb__after_unlock_lock() already works as advertised > > > with all our acquire/release thingies -- as is stated by the > > > documentation. > > > > > > That said, I'm not aware of anybody but RCU actually using this, so its > > > not used in that capacity. > > > > OK, I might actually understand what you are getting at. And, yes, if > > someone actually comes up with a need to combine smp_mb__after_unlock_lock() > > with something other than locking, we should worry about it at that point. > > And probably rename smp_mb__after_unlock_lock() at that point, as well. > > Until then, why lock ourselves into semantics that no one needs, and > > that it is quite possible that no one will ever need? > > Given that RCU is currently the only user of this barrier, how would you > feel about making the barrier local to RCU and not part of the general > memory-barrier API? In theory, no objection. Your thought is to leave the definitions where they are, mark them as being used only by RCU, and removing mention from memory-barriers.txt? Or did you have something else in mind? > My main reason for proposing its removal is because I don't want to see > it being used (incorrectly) all over the place to order the new RELEASE > and ACQUIRE operations I posted separately, at which point we have to try > fixing up all the callers or retrofitting some semantics. It doesn't help > that memory-barriers.txt lumps things like LOCK and ACQUIRE together, > whereas this barrier is currently only intended to be used in conjunction > with the former. Heh! That lumping was considered to be a feature at the time. ;-) Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
