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

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Date: Thu, 22 Feb 2024 12:50:12 +0100
From: Toke Høiland-Jørgensen <toke@...hat.com>
To: Benjamin Tissoires <bentiss@...nel.org>, Alexei Starovoitov
 <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, John Fastabend
 <john.fastabend@...il.com>, Andrii Nakryiko <andrii@...nel.org>, Martin
 KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>,
 Song Liu <song@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>, KP
 Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...gle.com>, Hao Luo
 <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, Jiri Kosina
 <jikos@...nel.org>, Benjamin Tissoires <benjamin.tissoires@...hat.com>,
 Jonathan Corbet <corbet@....net>, Shuah Khan <shuah@...nel.org>
Cc: bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-input@...r.kernel.org, linux-doc@...r.kernel.org,
 linux-kselftest@...r.kernel.org, Benjamin Tissoires <bentiss@...nel.org>
Subject: Re: [PATCH RFC bpf-next v3 04/16] bpf/helpers: introduce sleepable
 bpf_timers

Benjamin Tissoires <bentiss@...nel.org> 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