Index: kernel/generic/src/synch/waitq.c
===================================================================
--- kernel/generic/src/synch/waitq.c	(revision 96b02eb9b2f96f3843b8275c254c43a9cb6c8c88)
+++ kernel/generic/src/synch/waitq.c	(revision caa8a94d13e5b8fa72a19fe1bcb1d82be5482739)
@@ -127,6 +127,10 @@
 /** Interrupt sleeping thread.
  *
- * This routine attempts to interrupt a thread from its sleep in a waitqueue.
- * If the thread is not found sleeping, no action is taken.
+ * This routine attempts to interrupt a thread from its sleep in
+ * a waitqueue. If the thread is not found sleeping, no action
+ * is taken.
+ *
+ * The threads_lock must be already held and interrupts must be
+ * disabled upon calling this function.
  *
  * @param thread Thread to be interrupted.
@@ -138,7 +142,8 @@
 	DEADLOCK_PROBE_INIT(p_wqlock);
 	
-	irq_spinlock_lock(&threads_lock, true);
-	if (!thread_exists(thread))
-		goto out;
+	/*
+	 * The thread is quaranteed to exist because
+	 * threads_lock is held.
+	 */
 	
 grab_locks:
@@ -150,14 +155,13 @@
 			/*
 			 * The sleep cannot be interrupted.
-			 *
 			 */
 			irq_spinlock_unlock(&thread->lock, false);
-			goto out;
+			return;
 		}
 		
 		if (!irq_spinlock_trylock(&wq->lock)) {
+			/* Avoid deadlock */
 			irq_spinlock_unlock(&thread->lock, false);
 			DEADLOCK_PROBE(p_wqlock, DEADLOCK_THRESHOLD);
-			/* Avoid deadlock */
 			goto grab_locks;
 		}
@@ -173,11 +177,9 @@
 		irq_spinlock_unlock(&wq->lock, false);
 	}
+	
 	irq_spinlock_unlock(&thread->lock, false);
 	
 	if (do_wakeup)
 		thread_ready(thread);
-	
-out:
-	irq_spinlock_unlock(&threads_lock, true);
 }
 
@@ -370,5 +372,4 @@
 		 * If the thread was already interrupted,
 		 * don't go to sleep at all.
-		 *
 		 */
 		if (THREAD->interrupted) {
@@ -381,5 +382,4 @@
 		 * Set context that will be restored if the sleep
 		 * of this thread is ever interrupted.
-		 *
 		 */
 		THREAD->sleep_interruptible = true;