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). Similarly the store before osq_wait_next() in osq_lock(), namely: cmpxchg(&prev->node, node, NULL) is fully ordered. So I cannot see any store being delayed past the atomic_cmpxchg_acquire(). Now you mention 'compare and swap inside osq_lock' which I take to be the latter; and it _is_ fully ordered. -- 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/