Messages in this thread | | | Date | Wed, 26 Jun 2019 12:35:58 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH RT v2] Fix a lockup in wait_for_completion() and friends |
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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.
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