Re: [PATCH 1/2] getrusage: move thread_group_cputime_adjusted() outside of lock_task_sighand()

[Dylan Hatch]

On Fri, Jan 19, 2024 at 6:16 AM Oleg Nesterov <oleg@xxxxxxxxxx> 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.
>
> This is also preparation for the next patch which changes getrusage() to
> use stats_lock instead of siglock. Currently the deadlock is not possible,
> if getrusage() enters the slow path and takes stats_lock, read_seqretry()
> in thread_group_cputime() must always return 0, so thread_group_cputime()
> will never try to take the same lock. Yet this looks more safe and better
> performance-wise.
>
> Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> ---
>  kernel/sys.c | 34 +++++++++++++++++++---------------
>  1 file changed, 19 insertions(+), 15 deletions(-)
>
> diff --git a/kernel/sys.c b/kernel/sys.c
> index e219fcfa112d..70ad06ad852e 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -1785,17 +1785,19 @@ void getrusage(struct task_struct *p, int who, struct rusage *r)
>         struct task_struct *t;
>         unsigned long flags;
>         u64 tgutime, tgstime, utime, stime;
> -       unsigned long maxrss = 0;
> +       unsigned long maxrss;
> +       struct mm_struct *mm;
>         struct signal_struct *sig = p->signal;
>
> -       memset((char *)r, 0, sizeof (*r));
> +       memset(r, 0, sizeof(*r));
>         utime = stime = 0;
> +       maxrss = 0;
>
>         if (who == RUSAGE_THREAD) {
>                 task_cputime_adjusted(current, &utime, &stime);
>                 accumulate_thread_rusage(p, r);
>                 maxrss = sig->maxrss;
> -               goto out;
> +               goto out_thread;
>         }
>
>         if (!lock_task_sighand(p, &flags))
> @@ -1819,9 +1821,6 @@ void getrusage(struct task_struct *p, int who, struct rusage *r)
>                 fallthrough;
>
>         case RUSAGE_SELF:
> -               thread_group_cputime_adjusted(p, &tgutime, &tgstime);
> -               utime += tgutime;
> -               stime += tgstime;
>                 r->ru_nvcsw += sig->nvcsw;
>                 r->ru_nivcsw += sig->nivcsw;
>                 r->ru_minflt += sig->min_flt;
> @@ -1839,19 +1838,24 @@ void getrusage(struct task_struct *p, int who, struct rusage *r)
>         }
>         unlock_task_sighand(p, &flags);
>
> -out:
> -       r->ru_utime = ns_to_kernel_old_timeval(utime);
> -       r->ru_stime = ns_to_kernel_old_timeval(stime);
> +       if (who == RUSAGE_CHILDREN)
> +               goto out_children;
>
> -       if (who != RUSAGE_CHILDREN) {
> -               struct mm_struct *mm = get_task_mm(p);
> +       thread_group_cputime_adjusted(p, &tgutime, &tgstime);
> +       utime += tgutime;
> +       stime += tgstime;
>
> -               if (mm) {
> -                       setmax_mm_hiwater_rss(&maxrss, mm);
> -                       mmput(mm);
> -               }
> +out_thread:
> +       mm = get_task_mm(p);
> +       if (mm) {
> +               setmax_mm_hiwater_rss(&maxrss, mm);
> +               mmput(mm);
>         }
> +
> +out_children:
>         r->ru_maxrss = maxrss * (PAGE_SIZE / 1024); /* convert pages to KBs */
> +       r->ru_utime = ns_to_kernel_old_timeval(utime);
> +       r->ru_stime = ns_to_kernel_old_timeval(stime);
>  }
>
>  SYSCALL_DEFINE2(getrusage, int, who, struct rusage __user *, ru)
> --
> 2.25.1.362.g51ebf55
>
>

Tested-by: Dylan Hatch <dylanbhatch@xxxxxxxxxx>