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SubjectRe: [PATCH] kernel: release ptraced tasks before zap_pid_ns_processes
Sorry for delay, vacation,

On 01/02, Andrei Vagin wrote:
>
> zap_pid_ns_processes() can stuck on waiting tasks from the dead list. In
> this case, we will have one unkillable process with one or more dead
> children.

Thanks!

> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -664,9 +664,6 @@ static void forget_original_parent(struct task_struct *father,
> {
> struct task_struct *p, *t, *reaper;
>
> - if (unlikely(!list_empty(&father->ptraced)))
> - exit_ptrace(father, dead);
> -
> /* Can drop and reacquire tasklist_lock */
> reaper = find_child_reaper(father);
> if (list_empty(&father->children))
> @@ -705,8 +702,18 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
> LIST_HEAD(dead);
>
> write_lock_irq(&tasklist_lock);
> - forget_original_parent(tsk, &dead);
> + if (unlikely(!list_empty(&tsk->ptraced)))
> + exit_ptrace(tsk, &dead);
> + write_unlock_irq(&tasklist_lock);
> +
> + /* Ptraced tasks have to be released before zap_pid_ns_processes(). */
> + list_for_each_entry_safe(p, n, &dead, ptrace_entry) {
> + list_del_init(&p->ptrace_entry);
> + release_task(p);
> + }
>
> + write_lock_irq(&tasklist_lock);
> + forget_original_parent(tsk, &dead);
> if (group_dead)
> kill_orphaned_pgrp(tsk->group_leader, NULL);

How about a different fix below? It avoids additional write_lock/unlock(tasklist),
and another list_for_each_entry_safe(dead) loop is called only if it is actually
needed.

Or I missed something?

Oleg.


--- x/kernel/exit.c
+++ x/kernel/exit.c
@@ -558,12 +558,14 @@ static struct task_struct *find_alive_th
return NULL;
}

-static struct task_struct *find_child_reaper(struct task_struct *father)
+static struct task_struct *find_child_reaper(struct task_struct *father,
+ struct list_head *dead)
__releases(&tasklist_lock)
__acquires(&tasklist_lock)
{
struct pid_namespace *pid_ns = task_active_pid_ns(father);
struct task_struct *reaper = pid_ns->child_reaper;
+ struct task_struct *p, *n;

if (likely(reaper != father))
return reaper;
@@ -579,6 +581,11 @@ static struct task_struct *find_child_re
panic("Attempted to kill init! exitcode=0x%08x\n",
father->signal->group_exit_code ?: father->exit_code);
}
+
+ list_for_each_entry_safe(p, n, &dead, ptrace_entry) {
+ list_del_init(&p->ptrace_entry);
+ release_task(p);
+ }
zap_pid_ns_processes(pid_ns);
write_lock_irq(&tasklist_lock);

@@ -668,7 +675,7 @@ static void forget_original_parent(struc
exit_ptrace(father, dead);

/* Can drop and reacquire tasklist_lock */
- reaper = find_child_reaper(father);
+ reaper = find_child_reaper(father, dead);
if (list_empty(&father->children))
return;

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