On Tue, Nov 26, 2013 at 10:59:45AM +0100, Peter Zijlstra wrote: > On Mon, Nov 25, 2013 at 03:52:52PM -0800, Paul E. McKenney wrote: > > On Mon, Nov 25, 2013 at 07:27:15PM +0100, Peter Zijlstra wrote: > > > On Mon, Nov 25, 2013 at 10:02:50AM -0800, Paul E. McKenney wrote: > > > > And if the two locks are different, then the guarantee applies only > > > > when the unlock and lock are on the same CPU, in which case, as Linus > > > > noted, the xchg() on entry to the slow path does the job for use. > > > > > > But in that case we rely on the fact that the thing is part of a > > > composite and we should no longer call it load_acquire, because frankly > > > it doesn't have acquire semantics anymore because the read can escape > > > out. > > > > Actually, load-acquire and store-release are only required to provide > > ordering in the threads/CPUs doing the load-acquire/store-release > > operations. It is just that we require something stronger than minimal > > load-acquire/store-release to make a Linux-kernel lock. > > I suspect we're talking past one another here; but our Document > describes ACQUIRE/RELEASE semantics such that > > RELEASE > ACQUIRE > > matches a full barrier, regardless on whether it is the same lock or > not. Ah, got it! > If you now want to weaken this definition, then that needs consideration > because we actually rely on things like > > spin_unlock(l1); > spin_lock(l2); > > being full barriers. > > Now granted, for lock operations we have actual atomic ops in between > which would cure x86, but it would leave us confused with the barrier > semantics. > > So please; either: > > A) we have the strong ACQUIRE/RELEASE semantics as currently described; > and therefore any RELEASE+ACQUIRE pair must form a full barrier; and > our propose primitives are non-compliant and needs strengthening. > > B) we go fudge about with the definitions. Another approach would be to have local and global variants, so that the local variants have acquire/release semantics that are guaranteed to be visible only in the involved threads (sufficient for circular buffers) while the global ones are visible globally, thus sufficient for queued locks. > But given the current description of our ACQUIRE barrier, we simply > cannot claim the proposed primitives are good on x86 IMO. > > Also, instead of the smp_store_release() I would argue that > smp_load_acquire() is the one that needs the full buffer, even on PPC. > > Because our ACQUIRE dis-allows loads/stores leaking out upwards, and > both TSO and PPC lwsync allow just that, so the smp_load_acquire() is > the one that needs the full barrier. You lost me on this one. Here is x86 ACQUIRE for X: r1 = ACCESS_ONCE(X); <loads and stores> Since x86 does not reorder loads with later loads or stores, this should be sufficience. For powerpc: r1 = ACCESS_ONCE(X); lwsync; <loads and stores> And lwsync does not allow prior loads to be reordered with later loads or stores, so this should also be sufficient. In both cases, a RELEASE+ACQUIRE provides a full barrier as long as RELEASE has the right stuff in it. So what am I missing? Thanx, Paul -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>