-NOTE: this patch is a rebase of John Blackwood's patch. On his kernel, he must be using 
-an older simple wait patch - as his applies to kernel/sched/core.c, while the simple wait
-completion code lives in kernel/sched/completion.c ... I have ported this to test with 
-nvidia, as i would like to see if it fixes the semaphore issues i have seen. 

-I've kept the original patch comment in tact;

I'm not 100% sure that the patch below will fix your problem, but we
saw something that sounds pretty familiar to your issue involving the
nvidia driver and the preempt-rt patch.  The nvidia driver uses the
completion support to create their own driver's notion of an internally
used semaphore.

Fix a race in the PRT wait for completion simple wait code. 

A wait_for_completion() waiter task can be awoken by a task calling
complete(), but fail to consume the 'done' completion resource if it
looses a race with another task calling wait_for_completion() just as
it is waking up.

In this case, the awoken task will call schedule_timeout() again
without being in the simple wait queue.

So if the awoken task is unable to claim the 'done' completion resource,
check to see if it needs to be re-inserted into the wait list before
waiting again in schedule_timeout().

Fix-by: John Blackwood <john.blackwood@ccur.com>

--- linux-3.14/kernel/sched/completion.c	2014-05-22 14:01:03.879734869 -0400
+++ linux-3.14/kernel/sched/completion.c	2014-05-22 14:13:59.181688658 -0400
@@ -61,11 +61,19 @@
 do_wait_for_common(struct completion *x,
 		   long (*action)(long), long timeout, int state)
 {
+        int again = 0;
+
 	if (!x->done) {
 		DEFINE_SWAITER(wait);
 
 		swait_prepare_locked(&x->wait, &wait);
 		do {
+                       /* Check to see if we lost race for 'done' and are
+                        * no longer in the wait list.
+                        */
+                       if (unlikely(again) && list_empty(&wait.node))
+                               swait_prepare_locked(&x->wait, &wait);
+
 			if (signal_pending_state(state, current)) {
 				timeout = -ERESTARTSYS;
 				break;
@@ -74,6 +82,7 @@
 			raw_spin_unlock_irq(&x->wait.lock);
 			timeout = action(timeout);
 			raw_spin_lock_irq(&x->wait.lock);
+                        again = 1;
 		} while (!x->done && timeout);
 		swait_finish_locked(&x->wait, &wait);
 		if (!x->done)