[PATCH RT 5/6] sched/completion: Fix a lockup in wait_for_completion()

[Steven Rostedt]

4.19.37-rt20-rc1 stable review patch.
If anyone has any objections, please let me know.

------------------

From: Corey Minyard <cminyard@xxxxxxxxxx>

Consider following race:

  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 the thread to the wait queue inside the do loop.
The design of swait detects if it is already in the list and doesn't
do the list add again.

Cc: stable-rt@xxxxxxxxxxxxxxx
Fixes: a04ff6b4ec4ee7e ("completion: Use simple wait queues")
Signed-off-by: Corey Minyard <cminyard@xxxxxxxxxx>
Acked-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
[bigeasy: shorten commit message ]
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
 kernel/sched/completion.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/completion.c b/kernel/sched/completion.c
index 755a58084978..49c14137988e 100644
--- a/kernel/sched/completion.c
+++ b/kernel/sched/completion.c
@@ -72,12 +72,12 @@ do_wait_for_common(struct completion *x,
 	if (!x->done) {
 		DECLARE_SWAITQUEUE(wait);
 
-		__prepare_to_swait(&x->wait, &wait);
 		do {
 			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);
-- 
2.20.1