Re: [PATCH RFC bpf-next v3 04/16] bpf/helpers: introduce sleepable bpf_timers

[Toke Høiland-Jørgensen]

Benjamin Tissoires <bentiss@xxxxxxxxxx> writes:

> @@ -1245,6 +1294,7 @@ BPF_CALL_3(bpf_timer_set_callback, struct bpf_timer_kern *, timer, void *, callb
>  		ret = -EPERM;
>  		goto out;
>  	}
> +	down(&t->sleepable_lock);
>  	prev = t->prog;
>  	if (prev != prog) {
>  		/* Bump prog refcnt once. Every bpf_timer_set_callback()
> @@ -1261,6 +1311,7 @@ BPF_CALL_3(bpf_timer_set_callback, struct bpf_timer_kern *, timer, void *, callb
>  		t->prog = prog;
>  	}
>  	rcu_assign_pointer(t->callback_fn, callback_fn);
> +	up(&t->sleepable_lock);
>  out:
>  	__bpf_spin_unlock_irqrestore(&timer->lock);
>  	return ret;
> @@ -1282,7 +1333,7 @@ BPF_CALL_3(bpf_timer_start, struct bpf_timer_kern *, timer, u64, nsecs, u64, fla
>  
>  	if (in_nmi())
>  		return -EOPNOTSUPP;
> -	if (flags & ~(BPF_F_TIMER_ABS | BPF_F_TIMER_CPU_PIN))
> +	if (flags & ~(BPF_F_TIMER_ABS | BPF_F_TIMER_CPU_PIN | BPF_F_TIMER_SLEEPABLE))
>  		return -EINVAL;
>  	__bpf_spin_lock_irqsave(&timer->lock);
>  	t = timer->timer;
> @@ -1299,7 +1350,10 @@ BPF_CALL_3(bpf_timer_start, struct bpf_timer_kern *, timer, u64, nsecs, u64, fla
>  	if (flags & BPF_F_TIMER_CPU_PIN)
>  		mode |= HRTIMER_MODE_PINNED;
>  
> -	hrtimer_start(&t->timer, ns_to_ktime(nsecs), mode);
> +	if (flags & BPF_F_TIMER_SLEEPABLE)
> +		schedule_work(&t->work);
> +	else
> +		hrtimer_start(&t->timer, ns_to_ktime(nsecs), mode);
>  out:
>  	__bpf_spin_unlock_irqrestore(&timer->lock);
>  	return ret;

I think it's a little weird to just ignore the timeout parameter when
called with the sleepable flag. But I guess it can work at least as a
first pass; however, in that case we should enforce that the caller
passes in a timeout of 0, so that if we do add support for a timeout for
sleepable timers in the future, callers will be able to detect this.

-Toke