Re: [rfc][patch] Avoid taking global tasklist_lock for single threadedprocess at getrusage()

[Oleg Nesterov]

Sorry for delay,

Ravikiran G Thirumalai wrote:
> 
>  static void k_getrusage(struct task_struct *p, int who, struct rusage *r)
> @@ -1681,14 +1697,22 @@ static void k_getrusage(struct task_stru
>         struct task_struct *t;
>         unsigned long flags;
>         cputime_t utime, stime;
> +       int need_lock = 0;

Unneeded initialization

>         memset((char *) r, 0, sizeof *r);
> -
> -       if (unlikely(!p->signal))
> -               return;
> -
>         utime = stime = cputime_zero;
> 
> +       need_lock = !(p == current && thread_group_empty(p));
> +       if (need_lock) {
> +               read_lock(&tasklist_lock);
> +               if (unlikely(!p->signal)) {
> +                       read_unlock(&tasklist_lock);
> +                       return;
> +               }
> +       } else
> +               /* See locking comments above */
> +               smp_rmb();

This patch doesn't try to optimize ->sighand.siglock locking,
and I think this is right. But this also means we don't need
rmb() here. It was needed to protect against "another thread
just exited, cpu can read ->c* values before thread_group_empty()
without taking siglock" case, now it is not possible.

Oleg.
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