Re: [RFC,PATCH] Use RCU to protect tasklist for unicast signals

[Oleg Nesterov <oleg@tv-sign.ru>]

Paul E. McKenney wrote:
>
> On Wed, Aug 17, 2005 at 06:35:03PM +0400, Oleg Nesterov wrote:
> >
> > Sorry, I don't understand you. CLONE_THREAD implies CLONE_SIGHAND,
> > so we always need to lock one ->sighand. Could you please clarify?
>
> On the #3 and #4 code paths, the code assumes that the task-list lock
> is held.  So I was thinking of something (very roughly) as follows:
>
> #define SIGLOCK_HOLD_RCU      (1 << 0)
> #define SIGLOCK_HOLD_TASKLIST (1 << 1)
> #define SIGLOCK_HOLD_SIGLOCK  (1 << 2)

Oh, no, sorry for confusion.

I meant this function should only lock ->sighand, nothing more, something
like this:

// must be called with preemtion disabled !!!
struct sighand_struct *lock_task_sighand(struct task_struct *tsk, unsigned long *flags)
{
	struct sighand_struct *sighand;

//	sighand = NULL;
//	if (!get_task_struct_rcu(tsk))
//		goto out;

	for (;;) {
		sighand = tsk->sighand;
		if (unlikely(sighand == NULL))
			break;

		spin_lock_irqsave(sighand->siglock, *flags);

		if (likely(sighand == tsk->sighand)
			goto out;

		spin_unlock_irqrestore(sighand->siglock, *flags);
	}

//	put_task_struct(tsk);
out:
	return sighand;
}

static inline void unlock_task_sighand(struct task_struct *tsk, unsigned long *flags)
{
	spin_unlock_irqrestore(tsk->sighand->siglock, flags);
//	put_task_struct(tsk);
}

int group_send_sig_info(int sig, struct siginfo *info, struct task_struct *p)
{
	unsigned long flags;
	int ret;

	ret = check_kill_permission(sig, info, p);
	if (!ret && sig) {
		ret = -ESRCH;
		if (lock_task_sighand(p, &flags)) {
			ret = __group_send_sig_info(sig, info, p);
			unlock_task_sighand(p, &flags);
		}
	}

	return ret;
}

Currently the only user of it will be group_send_sig_info(), but I hope
you have devil plans to kill the "tasklist_lock guards the very rare
->sighand change" finally :)

Oleg.