[PATCH] correct use_mm()/unuse_mm() to use task_lock() to protect ->mm

From: William Lee Irwin III <wli@holomorphy.com>

Split off from suparna's patches:

Correct use_mm()/unuse_mm() to use task_lock() to protect task->mm.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/aio.c

@@ -538,19 +538,25 @@ struct kioctx *lookup_ioctx(unsigned long ctx_id)
 
 static void use_mm(struct mm_struct *mm)
 {
-	struct mm_struct *active_mm = current->active_mm;
+	struct mm_struct *active_mm;
+
 	atomic_inc(&mm->mm_count);
+	task_lock(current);
+	active_mm = current->active_mm;
 	current->mm = mm;
 	if (mm != active_mm) {
 		current->active_mm = mm;
 		activate_mm(active_mm, mm);
 	}
+	task_unlock(current);
 	mmdrop(active_mm);
 }
 
 static void unuse_mm(struct mm_struct *mm)
 {
+	task_lock(current);
 	current->mm = NULL;
+	task_unlock(current);
 	/* active_mm is still 'mm' */
 	enter_lazy_tlb(mm, current);
 }