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