Hi all, On Fri, Dec 11, 2015 at 09:41:33AM +0100, Peter Zijlstra wrote: > On Thu, Dec 10, 2015 at 08:51:34PM -0800, Andrew Pinski wrote: > > > So looking further I think I understand what is going wrong and why > > c55a6ffa6285e29f874ed403979472631ec70bff is incorrect. > > The osq_wait_next() call in osq_lock() is when we fail the lock. This is > effectively trylock() semantics and like for cmpxchg a failed trylock > has no implied barrier semantics. So from that POV osq_wait_next() does > not need to provide ACQUIRE semantics. > > In osq_unlock() there's an xchg() in front, which implies full barriers > and thereby provides RELEASE semantics for that part of osq_unlock(), so > again, from this POV osq_wait_next() does not need to provide RELEASE > semantics. > > > The compare and swap inside osq_lock needs to be both release and > > acquire semantics memory barriers because the stores (to node) need to > > be visible to the other cores before the setting of lock->tail > > happens. > > I'm a wee bit confused on what exactly you mean. Both stores to @node: > > 1) osq_wait_next(): next = xchg(&node->next, NULL) > 2) osq_unlock(): next = xchg(&node->next, NULL) > > are xchg() calls which imply full ordering (sequential consistency).
I think Andrew meant the atomic_xchg_acquire at the start of osq_lock, as opposed to "compare and swap". In which case, it does look like there's a bug here because there is nothing to order the initialisation of the node fields with publishing of the node, whether that's indirectly as a result of setting the tail to the current CPU or directly as a result of the WRITE_ONCE. Andrew, David: does making that atomic_xchg_acquire and atomic_xchg fix things for you? I don't fully grok what 81a43adae3b9 has to do with any of this, so maybe there's another bug too. Will --->8 diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c index d092a0c9c2d4..05a37857ab55 100644 --- a/kernel/locking/osq_lock.c +++ b/kernel/locking/osq_lock.c @@ -93,10 +93,12 @@ bool osq_lock(struct optimistic_spin_queue *lock) node->cpu = curr; /* - * ACQUIRE semantics, pairs with corresponding RELEASE - * in unlock() uncontended, or fastpath. + * We need both ACQUIRE (pairs with corresponding RELEASE in + * unlock() uncontended, or fastpath) and RELEASE (to publish + * the node fields we just initialised) semantics when updating + * the lock tail. */ - old = atomic_xchg_acquire(&lock->tail, curr); + old = atomic_xchg(&lock->tail, curr); if (old == OSQ_UNLOCKED_VAL) return true; -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/