Subject: FW: Commit 81a43adae3b9 (locking/mutex: Use acquire/release semantics) causing failures on arm64 (ThunderX)

From: [email-masked] (Peter Zijlstra)
Date: Fri, 11 Dec 2015 09:41:33 +0100

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.