linux-kernel - Re: [PATCH 1/3] fs/proc: do_task_stat: move thread_group_cputime_adjusted() outside of lock_task

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Date: Tue, 23 Jan 2024 15:48:15 -0800
From: Dylan Hatch <dylanbhatch@...gle.com>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, "Eric W. Biederman" <ebiederm@...ssion.com>, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] fs/proc: do_task_stat: move thread_group_cputime_adjusted()
 outside of lock_task_sighand()

On Tue, Jan 23, 2024 at 7:35 AM Oleg Nesterov <oleg@...hat.com> wrote:
>
> thread_group_cputime() does its own locking, we can safely shift
> thread_group_cputime_adjusted() which does another for_each_thread loop
> outside of ->siglock protected section.
>
> Not only this removes for_each_thread() from the critical section with
> irqs disabled, this removes another case when stats_lock is taken with
> siglock held. We want to remove this dependency, then we can change the
> users of stats_lock to not disable irqs.
>
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
> ---
>  fs/proc/array.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/fs/proc/array.c b/fs/proc/array.c
> index ff08a8957552..45ba91863808 100644
> --- a/fs/proc/array.c
> +++ b/fs/proc/array.c
> @@ -511,7 +511,7 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
>
>         sigemptyset(&sigign);
>         sigemptyset(&sigcatch);
> -       cutime = cstime = utime = stime = 0;
> +       cutime = cstime = 0;
>         cgtime = gtime = 0;
>
>         if (lock_task_sighand(task, &flags)) {
> @@ -546,7 +546,6 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
>
>                         min_flt += sig->min_flt;
>                         maj_flt += sig->maj_flt;
> -                       thread_group_cputime_adjusted(task, &utime, &stime);
>                         gtime += sig->gtime;
>
>                         if (sig->flags & (SIGNAL_GROUP_EXIT | SIGNAL_STOP_STOPPED))
> @@ -562,10 +561,13 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
>
>         if (permitted && (!whole || num_threads < 2))
>                 wchan = !task_is_running(task);
> -       if (!whole) {
> +
> +       if (whole) {
> +               thread_group_cputime_adjusted(task, &utime, &stime);
> +       } else {
> +               task_cputime_adjusted(task, &utime, &stime);
>                 min_flt = task->min_flt;
>                 maj_flt = task->maj_flt;
> -               task_cputime_adjusted(task, &utime, &stime);
>                 gtime = task_gtime(task);
>         }
>
> --
> 2.25.1.362.g51ebf55
>

Signed-off-by: Dylan Hatch <dylanbhatch@...gle.com>