[JFFS2] Fix lack of locking in thread_should_wake()

The thread_should_wake() function trawls through the list of 'very
dirty' eraseblocks, determining whether the background GC thread should
wake. Doing this without holding the appropriate locks is a bad idea.

OLPC Trac #8615
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Cc: stable@kernel.org

fs/jffs2/background.c

@@ -85,15 +85,15 @@ static int jffs2_garbage_collect_thread(void *_c)
 	for (;;) {
 		allow_signal(SIGHUP);
 	again:
+		spin_lock(&c->erase_completion_lock);
 		if (!jffs2_thread_should_wake(c)) {
 			set_current_state (TASK_INTERRUPTIBLE);
+			spin_unlock(&c->erase_completion_lock);
 			D1(printk(KERN_DEBUG "jffs2_garbage_collect_thread sleeping...\n"));
-			/* Yes, there's a race here; we checked jffs2_thread_should_wake()
-			   before setting current->state to TASK_INTERRUPTIBLE. But it doesn't
-			   matter - We don't care if we miss a wakeup, because the GC thread
-			   is only an optimisation anyway. */
 			schedule();
-		}
+		} else
+			spin_unlock(&c->erase_completion_lock);
+			
 
 		/* This thread is purely an optimisation. But if it runs when
 		   other things could be running, it actually makes things a