On Wed, Sep 28, 2016 at 01:16:45PM +0200, Peter Zijlstra wrote: > On Wed, Sep 28, 2016 at 04:05:30AM -0700, Paul E. McKenney wrote: > > On Wed, Sep 28, 2016 at 09:05:46AM +0200, Peter Zijlstra wrote: > > > On Wed, Sep 28, 2016 at 03:06:21AM +1000, Nicholas Piggin wrote: > > > > On Tue, 27 Sep 2016 18:52:21 +0200 > > > > Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > > > > > > > > On Wed, Sep 28, 2016 at 12:53:18AM +1000, Nicholas Piggin wrote: > > > > > > The more interesting is the ability to avoid the barrier between fastpath > > > > > > clearing a bit and testing for waiters. > > > > > > > > > > > > unlock(): lock() (slowpath): > > > > > > clear_bit(PG_locked) set_bit(PG_waiter) > > > > > > test_bit(PG_waiter) test_bit(PG_locked) > > > > The point being that at least one of the test_bit() calls must return > > true? > > Yes, more or less. Either unlock() observes PG_waiters set, or lock() > observes PG_locked unset. (opposed to all our 'normal' examples the > initial state isn't all 0 and the stores aren't all 1 :-). You lost me on unlock() doing any observation, but yes, I transliterated to standard form, unintentionally, as it turns out. ;-) So the goal is that either test_bit(PG_waiter) sees the set_bit() or test_bit(PG_locked) sees the clear_bit(), correct? > > As far as I know, all architectures fully order aligned same-size > > machine-sized accesses to the same location even without barriers. > > In the example above, the PG_locked and PG_waiter are different bits in > > the same location, correct? (Looks that way, but the above also looks > > a bit abbreviated.) > > Correct, PG_* all live in the same word. That should make things somewhat more reliable. ;-) > > So unless they operate on the same location or are accompanied by > > something like the smp_mb__after_atomic() called out above, there > > is no ordering. > > Same word.. > > > > So I think you're right and that we can forgo the memory barriers here. > > > I even think this must be true on all architectures. > > > > > > Paul and Alan have a validation tool someplace, put them on Cc. > > > > It does not yet fully handle atomics yet (but maybe Alan is ahead of > > me here, in which case he won't be shy). However, the point about > > strong ordering of same-sized aligned accesses to a machine-sized > > location can be made without atomics: > > Great. That's what I remember from reading that stuff. ;-) 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>