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

[minyard]

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.

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@xxxxxxxxxx>
---
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);
 			__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;
 	}
-- 
2.17.1