Re: [PATCH -v9][RFC] mutex: implement adaptive spinning

Linus Torvalds
2009-01-13T16:16:19Z

On Tue, 13 Jan 2009, Peter Zijlstra wrote:
>
> Change mutex contention behaviour such that it will sometimes busy wait on
> acquisition - moving its behaviour closer to that of spinlocks.

Okey, dokey. Looks reasonable, but I wonder if this part came from v8 and
wasn't intentional:

> + if (atomic_xchg(&lock->count, -1) == 1) {
> + lock_acquired(&lock->dep_map, ip);
> + mutex_set_owner(lock);
> + preempt_enable();
> + return 0;
> + }

Now you're forcing the slow-path on unlock. Maybe it was intentional,
maybe it wasn't. Did you perhaps mean

if (atomic_cmpxchg(&lock->count, 1, 0) == 1) {

here? I thought we agreed it was safe, if only because it should be
equivalent to just having done "mutex_trylock()" instead of a "real" lock
sequence.

Linus