Re: [PATCH RT v2] Fix a lockup in wait_for_completion() and friends

[Peter Zijlstra <peterz@infradead.org>]

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.