LITMUS: don't let rt_domain deadlock in global plugins

On the T2000, GSN-EDF would eventually deadlock. This was caused by a deadlock
involving the release and ready locks of its rt_domain.

Usually, the dependency in GSN-EDF is:

	function		held  -> required
	=========================================
	enqueue: 		ready -> release
	gsnedf_job_release:	      -> ready
	arm_release_timer:	      -> release

So far so good. The problem arose when hrtimer detected that the timer being
armed had already expired (must have been just a few nanoseconds) and decided
to call the callback directly WHILE HOLDING THE RELEASE LOCK. Which led to the
following result:

	function		held    -> required	CPU
	====================================================
	enqueue: 		ready   -> release	5
	gsnedf_job_release:	release -> ready	6

We avoid this problem now by dropping the release lock in arm_release_timer()
before calling the job release callback.

1 files changed, 4 insertions, 3 deletions

diff --git a/litmus/rt_domain.c b/litmus/rt_domain.c
index 5df3100dfb..a90cb64f0f 100644
--- a/litmus/rt_domain.c
+++ b/litmus/rt_domain.c
@@ -66,18 +66,19 @@ static void arm_release_timers(unsigned long _rt)
 {
         rt_domain_t *rt = (rt_domain_t*) _rt;
         unsigned long flags;
+        struct list_head alt;
         struct list_head *pos, *safe;
         struct task_struct* t;
 
         spin_lock_irqsave(&rt->release_lock, flags);
+        list_replace_init(&rt->release_queue, &alt);
+        spin_unlock_irqrestore(&rt->release_lock, flags);
         
-        list_for_each_safe(pos, safe, &rt->release_queue) {
+        list_for_each_safe(pos, safe, &alt) {
                 t = list_entry(pos, struct task_struct, rt_list);
                 list_del(pos);
                 setup_job_release_timer(t);
         }
-
-        spin_unlock_irqrestore(&rt->release_lock, flags);
 }