[PATCH] avoid tasklist_lock at getrusage for multithreaded case too

 * fields when reaping, so a sample either gets all the additions of a

 * given child after it's reaped, or none so this sample is before reaping.

 *

 * tasklist_lock locking optimisation:

 * If we are current and single threaded, we do not need to take the tasklist

 * lock or the siglock.  No one else can take our signal_struct away,

 * no one else can reap the children to update signal->c* counters, and

 * no one else can race with the signal-> fields.

 * If we do not take the tasklist_lock, the signal-> fields could be read

 * out of order while another thread was just exiting. So we place a

 * read memory barrier when we avoid the lock.  On the writer side,

 * write memory barrier is implied in  __exit_signal as __exit_signal releases

 * the siglock spinlock after updating the signal-> fields.

 *

 * We don't really need the siglock when we access the non c* fields

 * of the signal_struct (for RUSAGE_SELF) even in multithreaded

 * case, since we take the tasklist lock for read and the non c* signal->

 * fields are updated only in __exit_signal, which is called with

 * tasklist_lock taken for write, hence these two threads cannot execute

 * concurrently.

 * Locking:

 * We need to take the siglock for CHILDEREN, SELF and BOTH

 * for  the cases current multithreaded, non-current single threaded

 * non-current multithreaded.  Thread traversal is now safe with

 * the siglock held.

 * Strictly speaking, we donot need to take the siglock if we are current and

 * single threaded,  as no one else can take our signal_struct away, no one

 * else can  reap the  children to update signal->c* counters, and no one else

 * can race with the signal-> fields. If we do not take any lock, the

 * signal-> fields could be read out of order while another thread was just

 * exiting. So we should  place a read memory barrier when we avoid the lock.

 * On the writer side,  write memory barrier is implied in  __exit_signal

 * as __exit_signal releases  the siglock spinlock after updating the signal->

 * fields. But we don't do this yet to keep things simple.

 *

 */

	struct task_struct *t;

	unsigned long flags;

	cputime_t utime, stime;

	int need_lock = 0;

	memset((char *) r, 0, sizeof *r);

	utime = stime = cputime_zero;

	if (p != current || !thread_group_empty(p))

		need_lock = 1;

	if (need_lock) {

		read_lock(&tasklist_lock);

		if (unlikely(!p->signal)) {

			read_unlock(&tasklist_lock);

	rcu_read_lock();

	if (!lock_task_sighand(p, &flags)) {

		rcu_read_unlock();

		return;

	}

	} else

		/* See locking comments above */

		smp_rmb();

	switch (who) {

		case RUSAGE_BOTH:

		case RUSAGE_CHILDREN:

			spin_lock_irqsave(&p->sighand->siglock, flags);

			utime = p->signal->cutime;

			stime = p->signal->cstime;

			r->ru_nvcsw = p->signal->cnvcsw;

			r->ru_nivcsw = p->signal->cnivcsw;

			r->ru_minflt = p->signal->cmin_flt;

			r->ru_majflt = p->signal->cmaj_flt;

			spin_unlock_irqrestore(&p->sighand->siglock, flags);

			if (who == RUSAGE_CHILDREN)

				break;

			BUG();

	}

	if (need_lock)

		read_unlock(&tasklist_lock);

	unlock_task_sighand(p, &flags);

	rcu_read_unlock();

	cputime_to_timeval(utime, &r->ru_utime);

	cputime_to_timeval(stime, &r->ru_stime);

}