linux-kernel - Re: [PATCH bpf-next 17/18] bpf: add bpf_wq

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Date: Thu, 18 Apr 2024 23:18:48 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Benjamin Tissoires <bentiss@...nel.org>
Cc: Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, 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>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>, 
	Jiri Olsa <jolsa@...nel.org>, Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>, 
	bpf@...r.kernel.org, linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH bpf-next 17/18] bpf: add bpf_wq_start

On Tue, Apr 16, 2024 at 04:08:30PM +0200, Benjamin Tissoires wrote:
> again, copy/paste from bpf_timer_start().
> 
> Signed-off-by: Benjamin Tissoires <bentiss@...nel.org>
> ---
>  kernel/bpf/helpers.c | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index e5c8adc44619..ed5309a37eda 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -2728,6 +2728,29 @@ __bpf_kfunc int bpf_wq_init(struct bpf_wq *wq, void *map, unsigned int flags)
>  	return __bpf_async_init(async, map, flags, BPF_ASYNC_TYPE_WQ);
>  }
>  
> +__bpf_kfunc int bpf_wq_start(struct bpf_wq *wq, unsigned int flags)
> +{
> +	struct bpf_async_kern *async = (struct bpf_async_kern *)wq;
> +	struct bpf_work *w;
> +	int ret = 0;
> +
> +	if (in_nmi())
> +		return -EOPNOTSUPP;
> +	if (flags)
> +		return -EINVAL;
> +	__bpf_spin_lock_irqsave(&async->lock);
> +	w = async->work;
> +	if (!w || !w->cb.prog) {
> +		ret = -EINVAL;
> +		goto out;
> +	}
> +
> +	schedule_work(&w->work);
> +out:
> +	__bpf_spin_unlock_irqrestore(&async->lock);

Looks like you're not adding wq_cancel kfunc in this patch set and
it's probably a good thing not to expose async cancel to bpf users,
since it's a foot gun.
Even when we eventually add wq_cancel_sync kfunc it will not be
removing a callback.
So we can drop spinlock here.
READ_ONCE of w and cb would be enough.
Since they cannot get back to NULL once init-ed and cb is set.