Re: [PATCH v2] locking/mutex: Prevent lock starvation when spinning is enabled

Peter Zijlstra
2016-08-19T00:49:19Z

On Thu, Aug 18, 2016 at 04:27:35PM +0200, Peter Zijlstra wrote:
> On Wed, Aug 10, 2016 at 11:44:08AM -0700, Jason Low wrote:
> > @@ -556,8 +604,12 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
> > * other waiters. We only attempt the xchg if the count is
> > * non-negative in order to avoid unnecessary xchg operations:
> > */
> > - if (atomic_read(&lock->count) >= 0 &&
> > + if ((!need_yield_to_waiter(lock) || wakeups > 1) &&
> > + atomic_read(&lock->count) >= 0 &&
> > (atomic_xchg_acquire(&lock->count, -1) == 1))
> > + if (wakeups > 1)
> > + clear_yield_to_waiter(lock);
> > +
> > break;
> >
> > /*
>
> There's some { } gone missing there...
>
> Also, I think I'll change it to avoid that extra wakeups > 1 condition..

Also, its broken, even if we should not trylock, we should still very
much xchg(-1) to mark the lock as having waiters.