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

[Corey Minyard]

On Thu, May 09, 2019 at 06:19:25PM +0200, Sebastian Andrzej Siewior wrote:
> 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.

Will do.  I'll add your diagram below, too.

> 
> On 2019-05-08 15:57:28 [-0500], minyard@xxxxxxx wrote:
> > From: Corey Minyard <cminyard@xxxxxxxxxx>
> > 
> > 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

Yes, exactly.  I was wondering if swait could be changed to not remove
the waiter, but that seemed like a bad idea.  It is an unusual semantic,
though.

I thought some more about this, wondering why everything isn't keeling
over because of this.  I'm guessing that just about everything using
completions has a single waiter, so it doesn't matter.  I just wrote
some code that has a bunch of waiters, so I hit it.

-corey

> 
> 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@xxxxxxxxxx>
> > ---
> > 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