On Mon, Jul 03, 2017 at 09:40:22AM -0700, Linus Torvalds wrote:
> 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).
Good to know, that does make things easier. ;-)
I am not certain that it is feasible to automatically recognize
non-assert/non-debugging use cases of spin_is_locked(), but there is
aways manual inspection.
> 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.
OK, so we should rework any cases where people are relying on acquisition
of one spin_lock() being ordered with a later spin_is_locked() on some
other lock by that same thread.
Thanx, Paul
