Re: kernel BUG at kernel/exit.c:792!

From: Ingo Molnar
Date: Wed Dec 03 2003 - 15:01:29 EST

On Wed, 3 Dec 2003, Linus Torvalds wrote:

> > But I don't understand the oops:
> > __exit_sighand clears current->sighand, and then in the next line
> > __unhash_process removes the thread from the task list. But that's under
> > write_lock_irq(&tasklist_lock), and get_tid_list runs under
> > read_lock(&tasklist_lock). It should be impossible that ->sighand is
> > NULL and the task is still listed in the task list.
> The /proc filesystem will keep pointers to processes alive, and can
> reach them even if the process is otherwise gone.
> This is why /proc ends up doing tests like "if (tsk->mm)" etc - because
> it literally can see processes after they are dead.

yes, and we start the 'task list search' at leader_task, which might be an
already unlinked task. So i'd suggest to use a variant of Srivatsa's fix
(attached below) - the extra explanation at this place cannot hurt i


--- linux/fs/proc/base.c.orig
+++ linux/fs/proc/base.c
@@ -1666,7 +1666,12 @@ static int get_tid_list(int index, unsig

index -= 2;
- do {
+ /*
+ * The starting point task (leader_task) might be an already
+ * unlinked task, which cannot be used to access the task-list
+ * via next_thread().
+ */
+ if (pid_alive(task)) do {
int tid = task->pid;
if (!pid_alive(task))
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