On Tue, Jan 20, 2015 at 09:34:43AM +0000, Peter Zijlstra wrote: > On Tue, Jan 13, 2015 at 04:33:54PM +0000, Will Deacon wrote: > > 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. Indeed, and I'd love to kill that, especially as its really confusing when we have other ACQUIRE/RELEASE functions (like your smp_* accessors) that do need explicit barriers for general RELEASE->ACQUIRE ordering. If people start using smp_mb__after_unlock_lock for *that*, then other architectures will need to implement it as a barrier and penalise their spinlocks in doing so. > > 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. I'm absolutely fine with that, I'd just like to make sure that it's documented so that people don't use smp_mb__after_unlock_lock() to order smp_store_release -> smp_load_acquire. I'll have a crack at a Documentation patch if you don't beat me to it... 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
