[PATCH] Fix semundo lock leakage

semundo->lock can leak if semundo->refcount goes from 2 to 1 while
another thread has it locked. This causes major problems for PREEMPT
kernels.

The simplest fix for now is to undo the single-thread optimization.

This bug was found via relentless testing by Dominik Karall.

Signed-off-by: Ingo Molnar <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

ipc/sem.c

···
 	struct sem_undo_list *undo_list;
 
 	undo_list = current->sysvsem.undo_list;
-	if ((undo_list != NULL) && (atomic_read(&undo_list->refcnt) != 1))
+	if (undo_list)
 		spin_lock(&undo_list->lock);
 }
 
···
 	struct sem_undo_list *undo_list;
 
 	undo_list = current->sysvsem.undo_list;
-	if ((undo_list != NULL) && (atomic_read(&undo_list->refcnt) != 1))
+	if (undo_list)
 		spin_unlock(&undo_list->lock);
 }
 
···
 		if (undo_list == NULL)
 			return -ENOMEM;
 		memset(undo_list, 0, size);
-		/* don't initialize unodhd->lock here.  It's done
-		 * in copy_semundo() instead.
-		 */
+		spin_lock_init(&undo_list->lock);
 		atomic_set(&undo_list->refcnt, 1);
 		current->sysvsem.undo_list = undo_list;
 	}
···
 		error = get_undo_list(&undo_list);
 		if (error)
 			return error;
-		if (atomic_read(&undo_list->refcnt) == 1)
-			spin_lock_init(&undo_list->lock);
 		atomic_inc(&undo_list->refcnt);
 		tsk->sysvsem.undo_list = undo_list;
 	} else