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

Peter Zijlstra
2016-08-18T14:27:53Z

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..