Re: [PATCH RFC 08/26] locking: Remove spin_unlock_wait() generic definitions

[Linus Torvalds]

On Mon, Jul 3, 2017 at 9:18 AM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:
>
> Agreed, and my next step is to look at spin_lock() followed by
> spin_is_locked(), not necessarily the same lock.

Hmm. Most (all?) "spin_is_locked()" really should be about the same
thread that took the lock (ie it's about asserts and lock debugging).

The optimistic ABBA avoidance pattern for spinlocks *should* be

    spin_lock(inner)
    ...
    if (!try_lock(outer)) {
           spin_unlock(inner);
           .. do them in the right order ..

so I don't think spin_is_locked() should have any memory barriers.

In fact, the core function for spin_is_locked() is arguably
arch_spin_value_unlocked() which doesn't even do the access itself.

                       Linus