Re: do_syslog/__down_trylock lockup in current BK

[Ingo Molnar]

On Mon, 9 Sep 2002, Linus Torvalds wrote:

> > the lockup is likely in the while loop - ie. zap_thread() not actually
> > reparenting a thread and thus causing an infinite loop - is that possible?
> 
> Hmm.. This patch changes the last argument of zap_thread() from a "1" to
> a "0" for the ptrace_children list. Was that intentional or a
> cut-and-paste error? [...]

cut-and-paste error. New patch attached. (with the assert added as well)

	Ingo

--- linux/kernel/exit.c.orig	Mon Sep  9 21:59:24 2002
+++ linux/kernel/exit.c	Mon Sep  9 22:38:44 2002
@@ -461,6 +461,8 @@
 		ptrace_unlink (p);
 
 		list_del_init(&p->sibling);
+		if (p->parent == father && p->parent == p->real_parent)
+			BUG();
 		p->parent = p->real_parent;
 		list_add_tail(&p->sibling, &p->parent->children);
 	}
@@ -493,7 +495,6 @@
 static void exit_notify(void)
 {
 	struct task_struct *t;
-	struct list_head *_p, *_n;
 
 	forget_original_parent(current);
 	/*
@@ -554,17 +555,16 @@
 		do_notify_parent(current, current->exit_signal);
 
 zap_again:
-	list_for_each_safe(_p, _n, &current->children)
-		zap_thread(list_entry(_p,struct task_struct,sibling), current, 0);
-	list_for_each_safe(_p, _n, &current->ptrace_children)
-		zap_thread(list_entry(_p,struct task_struct,ptrace_list), current, 1);
+	while (!list_empty(&current->children))
+		zap_thread(list_entry(current->children.next,struct task_struct,sibling), current, 0);
+	while (!list_empty(&current->ptrace_children))
+		zap_thread(list_entry(current->ptrace_children.next,struct task_struct,sibling), current, 1);
 	/*
 	 * zap_thread might drop the tasklist lock, thus we could
 	 * have new children queued back from the ptrace list into the
 	 * child list:
 	 */
-	if (unlikely(!list_empty(&current->children) ||
-			!list_empty(&current->ptrace_children)))
+	if (unlikely(!list_empty(&current->children)))
 		goto zap_again;
 	/*
 	 * No need to unlock IRQs, we'll schedule() immediately