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SubjectRe: [PATCH RT v2] Fix a lockup in wait_for_completion() and friends
Please:
- add some RT developers on Cc:
- add lkml
- use [PATCH RT] instead just [PATCH] so it is visible that you target
the RT tree.

On 2019-05-08 15:57:28 [-0500], minyard@acm.org wrote:
> From: Corey Minyard <cminyard@mvista.com>
>
> The function call do_wait_for_common() has a race condition that
> can result in lockups waiting for completions. Adding the thread
> to (and removing the thread from) the wait queue for the completion
> is done outside the do loop in that function. However, if the thread
> is woken up, the swake_up_locked() function will delete the entry
> from the wait queue. If that happens and another thread sneaks
> in and decrements the done count in the completion to zero, the
> loop will go around again, but the thread will no longer be in the
> wait queue, so there is no way to wake it up.
>
> Fix it by adding/removing the thread to/from the wait queue inside
> the do loop.

So you are saying:
T0 T1 T2
wait_for_completion()
do_wait_for_common()
__prepare_to_swait()
schedule()
complete()
x->done++ (0 -> 1)
raw_spin_lock_irqsave()
swake_up_locked() wait_for_completion()
wake_up_process(T0)
list_del_init()
raw_spin_unlock_irqrestore()
raw_spin_lock_irq(&x->wait.lock)
raw_spin_lock_irq(&x->wait.lock) x->done != UINT_MAX, 1 -> 0
return 1
raw_spin_unlock_irq(&x->wait.lock)
while (!x->done && timeout),
continue loop, not enqueued
on &x->wait

The difference compared to the non-swait based implementation is that
swake_up_locked() removes woken up tasks from the list while the other
implementation (wait_queue_entry based, default_wake_function()) does
not. Buh

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?

> Fixes: a04ff6b4ec4ee7e ("completion: Use simple wait queues")
> Signed-off-by: Corey Minyard <cminyard@mvista.com>
> ---
> I sent the wrong version of this, I had spotted this before but didn't
> fix it here. Adding the thread to the wait queue needs to come after
> the signal check. Sorry about the noise.
>
> kernel/sched/completion.c | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/sched/completion.c b/kernel/sched/completion.c
> index 755a58084978..4f9b4cc0c95a 100644
> --- a/kernel/sched/completion.c
> +++ b/kernel/sched/completion.c
> @@ -70,20 +70,20 @@ do_wait_for_common(struct completion *x,
> long (*action)(long), long timeout, int state)
> {
> if (!x->done) {
> - DECLARE_SWAITQUEUE(wait);
> -
> - __prepare_to_swait(&x->wait, &wait);

you can keep DECLARE_SWAITQUEUE remove just __prepare_to_swait()

> do {
> + DECLARE_SWAITQUEUE(wait);
> +
> if (signal_pending_state(state, current)) {
> timeout = -ERESTARTSYS;
> break;
> }
> + __prepare_to_swait(&x->wait, &wait);

add this, yes and you are done.

> __set_current_state(state);
> raw_spin_unlock_irq(&x->wait.lock);
> timeout = action(timeout);
> raw_spin_lock_irq(&x->wait.lock);
> + __finish_swait(&x->wait, &wait);
> } while (!x->done && timeout);
> - __finish_swait(&x->wait, &wait);
> if (!x->done)
> return timeout;
> }

Sebastian

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