Re: + aio-completion-signal-notification.patch added to -mm tree

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

Sebastien Dugue wrote:
>
> +static long aio_setup_sigevent(struct aio_notify *notify,
> +			       struct sigevent __user *user_event)
> +{
> +	sigevent_t event;
> +	struct task_struct *target;
> +
> +	if (copy_from_user(&event, user_event, sizeof(event)))
> +		return -EFAULT;
> +
> +	if (event.sigev_notify == SIGEV_NONE)
> +		return 0;
> +
> +	notify->notify = event.sigev_notify;
> +	notify->signo = event.sigev_signo;
> +	notify->value = event.sigev_value;
> +
> +	read_lock(&tasklist_lock);

We don't need tasklist_lock, we can use rcu_read_lock() instead.

> +	target = good_sigevent(&event);
> +
> +	if (unlikely(!target || (target->flags & PF_EXITING)))
> +		goto out_unlock;

PF_EXITING check is racy and unneded. In fact, it is wrong. If the main
thread is already died, we can only use SIGEV_THREAD_ID signals, because
otherwise good_sigevent() returns ->group_leader.

> @@ -994,6 +1077,15 @@ int fastcall aio_complete(struct kiocb *
>  	kunmap_atomic(ring, KM_IRQ1);
>
>  	pr_debug("added to ring %p at [%lu]\n", iocb, tail);
> +
> +	if (iocb->ki_notify.notify != SIGEV_NONE) {
> +		ret = aio_send_signal(&iocb->ki_notify);
> +
> +		/* If signal generation failed, release the sigqueue */
> +		if (ret)
> +			sigqueue_free(iocb->ki_notify.sigq);

We should not use sigqueue_free() here. It takes current->sighand->siglock
to remove sigqueue from "struct sigpending". But current is just a "random"
process here.

Yes, if I understand this patch correctly, it is not possible that this
sigqueue is pending, but still this is bad imho.

>  static void __sigqueue_free(struct sigqueue *q)
>  {
> -	if (q->flags & SIGQUEUE_PREALLOC)
> +	if (q->flags & SIGQUEUE_PREALLOC && q->info.si_code != SI_ASYNCIO)
>  		return;

Oh, this is not nice. Could we change send_sigqueue/send_group_sigqueue
instead ?

-	BUG_ON(!(q->flags & SIGQUEUE_PREALLOC));
+	BUG_ON(!(q->flags & SIGQUEUE_PREALLOC) && q->info.si_code != SI_ASYNCIO);

This way aio can use __sigqueue_alloc/__sigqueue_free directly and forget
about SIGQUEUE_PREALLOC.

On the other hand, imho this patch takes a wrong direction.

The purpose of SIGQUEUE_PREALLOC + send_sigqueue() is to re-use the same
sigqueue while sending a stream of signals. But in aio case we allocate
sigqueue to send only 1 signal, then it freed after the delivery like
the regular sigqueue. So what is the point?

I'd suggest to not use this interface. Just use group_send_sig_info() or
specific_send_sig_info(). Yes, this way we will do GFP_ATOMIC allocation
of sigqueue in interrupt context, but is this so bad in this case?

Oleg.