On Wed, 2013-11-20 at 09:14 -0800, Paul E. McKenney wrote: > On Wed, Nov 20, 2013 at 03:46:43PM +0000, Will Deacon wrote: > > Hi Paul, > > > > On Wed, Nov 20, 2013 at 03:31:23PM +0000, Paul E. McKenney wrote: > > > On Tue, Nov 19, 2013 at 05:37:43PM -0800, Tim Chen wrote: > > > > @@ -68,7 +72,12 @@ void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node) > > > > while (!(next = ACCESS_ONCE(node->next))) > > > > arch_mutex_cpu_relax(); > > > > } > > > > - ACCESS_ONCE(next->locked) = 1; > > > > - smp_wmb(); > > > > + /* > > > > + * Pass lock to next waiter. > > > > + * smp_store_release() provides a memory barrier to ensure > > > > + * all operations in the critical section has been completed > > > > + * before unlocking. > > > > + */ > > > > + smp_store_release(&next->locked, 1); > > > > > > However, there is one problem with this that I missed yesterday. > > > > > > Documentation/memory-barriers.txt requires that an unlock-lock pair > > > provide a full barrier, but this is not guaranteed if we use > > > smp_store_release() for unlock and smp_load_acquire() for lock. > > > At least one of these needs a full memory barrier. > > > > Hmm, so in the following case: > > > > Access A > > unlock() /* release semantics */ > > lock() /* acquire semantics */ > > Access B > > > > A cannot pass beyond the unlock() and B cannot pass the before the lock(). > > > > I agree that accesses between the unlock and the lock can be move across both > > A and B, but that doesn't seem to matter by my reading of the above. > > > > What is the problematic scenario you have in mind? Are you thinking of the > > lock() moving before the unlock()? That's only permitted by RCpc afaiu, > > which I don't think any architectures supported by Linux implement... > > (ARMv8 acquire/release is RCsc). > > If smp_load_acquire() and smp_store_release() are both implemented using > lwsync on powerpc, and if Access A is a store and Access B is a load, > then Access A and Access B can be reordered. > > Of course, if every other architecture will be providing RCsc implementations > for smp_load_acquire() and smp_store_release(), which would not be a bad > thing, then another approach is for powerpc to use sync rather than lwsync > for one or the other of smp_load_acquire() or smp_store_release(). Can we count on the xchg function in the beginning of mcs_lock to provide a memory barrier? It should provide an implicit memory barrier according to the memory-barriers document. Thanks. Tim > > 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>