On Thu, May 09, 2019 at 06:19:25PM +0200, Sebastian Andrzej Siewior wrote: > One question for the upstream completion implementation: > completion_done() returns true if there are no waiters. It acquires the > wait.lock to ensure that complete()/complete_all() is done. However, > once complete releases the lock it is guaranteed that the wake_up() (for > the waiter) occurred. The waiter task still needs to be remove itself > from the wait-queue before the completion can be removed. > Do I miss something? So you mean: init_completion(&done); wait_for_copmletion(&done) spin_lock() __add_wait_queue() spin_unlock() schedule() complete() completion_done() spin_lock() __remove_wait_queue() spin_unlock() Right? I think that boils down to that whenever you have multiple waiters, someone needs to be in charge of @done's lifetime. The case that matters is: DECLARE_COMPLETION_ONSTACK(done) while (!completion_done(&done)) cpu_relax(); Where there is but a single waiter, and that waiter is completion_done(). In that case it must not return early. Now, I've also seen a ton of code do: if (!completion_done(done)) complete(done); And that makes me itch... but I've not bothered to look into it hard enough.
