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SubjectRe: [patch 1/4] fs, proc: Introduce /proc/<pid>/task/<tid>/children entry v8
On Mon, 23 Jan 2012 18:20:37 +0400
Cyrill Gorcunov <gorcunov@openvz.org> wrote:

> When we do checkpoint of a task we need to know the list of children
> the task, has but there is no easy and fast way to generate reverse
> parent->children chain from arbitrary <pid> (while a parent pid is
> provided in "PPid" field of /proc/<pid>/status).
>
> So instead of walking over all pids in the system (creating one big process
> tree in memory, just to figure out which children a task has) -- we add
> explicit /proc/<pid>/task/<tid>/children entry, because the kernel already has
> this kind of information but it is not yet exported.
>
> This is a first level children, not the whole process tree.
>
> v2:
> - Kame suggested to use a separated /proc/<pid>/children entry
> instead of poking /proc/<pid>/status
> - Andew suggested to use rcu facility instead of locking
> tasklist_lock
> - Tejun pointed that non-seekable seq file might not be
> enough for tasks with large number of children
>
> v3:
> - To be on a safe side use %lu format for pid_t printing
>
> v4:
> - New line get printed when sequence ends not at seq->stop,
> a nit pointed by Tejun
> - Documentation update
> - tasklist_lock is back, Oleg pointed that ->children list
> is actually not rcu-safe
>
> v5:
> - Oleg suggested to make /proc/<pid>/task/<tid>/children
> instead of global /proc/<pid>/children, which eliminates
> hardness related to threads and children migration, and
> allows patch to be a way simplier.
>
> v6:
> - Drop ptrace_may_access tests, pids are can be found anyway
> so nothing to protect here.
> - Update comments and docs, pointed by Oleg.
>
> v7:
> - Use get_pid over proc-pid directly, to simplify
> code, pointed by Oleg.
>
> v8:
> - Obtain a starting pid from the proc's inode directly.
>
> Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
> Reviewed-by: Oleg Nesterov <oleg@redhat.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Pavel Emelyanov <xemul@parallels.com>
> Cc: Serge Hallyn <serge.hallyn@canonical.com>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

From viewpoint I played with seq_file, yesterday.

> +static void *children_seq_start(struct seq_file *seq, loff_t *pos)
> +{
> + return get_children_pid(seq->private, NULL, *pos);
> +}
> +
> +static void *children_seq_next(struct seq_file *seq, void *v, loff_t *pos)
> +{
> + struct pid *pid = NULL;
> +
> + pid = get_children_pid(seq->private, v, *pos + 1);
> + if (!pid)
> + seq_printf(seq, "\n");
> + put_pid(v);

Because seq_printf() may fail. This seems dangeorus.

If seq_printf() fails and returns NULL, "\n" will not be
printed out and user land parser will go wrong.

I think all seq_printf() should be handled in ->show().
(And you can use seq_putc() for "\n".)

Thanks,
-Kame




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