On Thu, Sep 20, 2018 at 06:08:32PM +0200, Peter Zijlstra wrote:
> Another approach might be to use something like:
>
> val = xchg_relaxed(&lock->locked_pending, _Q_PENDING_VAL |
> _Q_LOCKED_VAL);
> val |= atomic_read_acquire(&lock->val) & _Q_TAIL_MASK;
>
> combined with something like:
>
> /* 0,0,0 -> 0,1,1 - we won trylock */
> if (!(val & _Q_LOCKED_MASK)) {
That one doesn't actually work... let me think about this more.
> clear_pending(lock);
> return;
> }
>
> /* 0,0,1 -> 0,1,1 - we won pending */
> if (!(val & ~_Q_LOCKED_MASK)) {
> ...
> }
>
> /* *,0,1 -> *,1,1 - we won pending, but there's queueing */
> if (!(val & _Q_PENDING_VAL))
> clear_pending(lock);
>
> ...
>
>
> Hmmm?
