On Fri, Dec 11, 2015 at 12:04:19PM +0000, Will Deacon wrote: > 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.
Agreed, this does indeed look like a bug. If confirmed please write a shiny changelog and I'll queue asap. > 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/
