On Tue, Jan 20, 2015 at 09:35:45PM +0000, Paul E. McKenney wrote: > On Tue, Jan 20, 2015 at 10:34:43AM +0100, Peter Zijlstra wrote: > > On Tue, Jan 13, 2015 at 04:33:54PM +0000, Will Deacon wrote: > > > Hi Paul, > > > > > > I started dusting off a series I've been working to implement a relaxed > > > atomic API in Linux (i.e. things like atomic_read(v, ACQUIRE)) but I'm > > > having trouble making sense of the ordering semantics we have in mainline > > > today: > > > > > 2. Does smp_mb__after_unlock_lock order smp_store_release against > > > smp_load_acquire? Again, Documentation/memory-barriers.txt puts > > > these operations into the RELEASE and ACQUIRE classes respectively, > > > but since smp_mb__after_unlock_lock is a NOP everywhere other than > > > PowerPC, I don't think this is enforced by the current code. > > > > Yeah, wasn't Paul going to talk to Ben about that? PPC is the only arch > > that has the weak ACQUIRE/RELEASE for its spinlocks. > > I thought that you guys were going to propose something and we would see > what the reaction was. ;-) > > > > Most > > > architectures follow the pattern used by asm-generic/barrier.h: > > > > > > release: smp_mb(); STORE > > > acquire: LOAD; smp_mb(); > > > > > > which doesn't provide any release -> acquire ordering afaict. > > > > Only when combined on the same address, if the LOAD observes the result > > of the STORE we can guarantee the rest of the ordering. And if you > > build a locking primitive with them (or circular lists or whatnot) you > > have that extra condition. > > > > But yes, I see your argument that this implementation is weak like the > > PPC. > > A more complete example would be as follows: > > STOREs followed by release: smp_mb(); STORE A > acquire: LOAD A; smp_mb(); preceding LOADs > > If the LOAD A gets the value from the STORE A, then the LOADs following > the acquire are guaranteed to see the STOREs preceding the release. > > And yes, this really truly does work fine with weaker ordering. I agree, but if we consider the case where the acquire and the release are operating on *different* addresses, then the current Documentation would tell us to use smp_mb__after_unlock_lock to ensure ordering, which won't work on anything other than Power. Will -- 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
