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

[Steven Rostedt <rostedt@goodmis.org>]

On Thu,  9 May 2019 13:02:11 -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.
> 
> Visually, here's a diagram from Sebastian Andrzej Siewior:
>   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
>                                                   raw_spin_unlock_irq(&x->wait.lock)
>                                                   return 1
>    while (!x->done && timeout),
>    continue loop, not enqueued
>    on &x->wait
> 
> Basically, the problem is that the original wait queues used in
> completions did not remove the item from the queue in the wakeup
> function, but swake_up_locked() does.
> 
> Fix it by adding/removing the thread to/from the wait queue inside
> the do loop.
> 
> Fixes: a04ff6b4ec4ee7e ("completion: Use simple wait queues")
> Signed-off-by: Corey Minyard <cminyard@mvista.com>
> ---
> This looks like a fairly serious bug, I guess, but I've never seen a
> report on it before.
> 
> I found it because I have an out-of-tree feature (hopefully in tree some
> day) that takes a core dump of a running process without killing it.  It
> makes extensive use of completions, and the test code is fairly brutal.
> It didn't lock up on stock 4.19, but failed with the RT patches applied.
> 
> The funny thing is, I've never seen this test code fail before on earlier
> releases, but it locks up pretty reliably on 4.19-rt.  It looks like this
> bug goes back to at least the 4.4-rt kernel.  But we haven't received any
> customer reports of failures.  I'm guessing that almost all completion
> users have a single waiter, so you would never see this.
> 
> The feature and test are in a public tree if someone wants to try to
> reproduce this.  But hopefully this is pretty obvious with the explaination.
> 
> Also, you could put the DECLARE_SWAITQUEUE() outside the loop, I think,
> but maybe it's cleaner or safer to declare it in the loop?  If someone
> cares I can test it that way.
> 
> -corey
> 
>  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);
>  		do {
> +			DECLARE_SWAITQUEUE(wait);
> +
>  			if (signal_pending_state(state, current)) {
>  				timeout = -ERESTARTSYS;
>  				break;
>  			}
> +			__prepare_to_swait(&x->wait, &wait);

Sebastian mentioned that moving the __prepare_to_swait() was the only
change needed. Please do that.

Thanks!

-- Steve

>  			__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;
>  	}