linux-kernel - Re: [PATCH bpf-next 11/18] bpf: wq: add bpf_wq

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Date: Fri, 19 Apr 2024 17:55:23 +0200
From: Benjamin Tissoires <bentiss@...nel.org>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
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 <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, 
	"open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH bpf-next 11/18] bpf: wq: add bpf_wq_init

On Apr 19 2024, Alexei Starovoitov wrote:
> On Fri, Apr 19, 2024 at 8:12 AM Benjamin Tissoires <bentiss@...nel.org> wrote:
> >
> >
> > It's something I added while adding the tests. And some tests were passing
> > in case I was having a non sleepable callback. But if we have
> > bpf_rcu_read_lock(), we are all fine and can reduce the complexity.
> 
> Not quite following what was the issue.
> Since the verifier is unconditionally verifying such callback as sleepable
> the callback won't be able to access rcu pointers without doing
> explicit bpf_rcu_read_lock() first (and few other code patterns
> might be rejected), but that's a good thing.

Oh, I missed that. Well, given that the verifier enforces everything, I
guess we are good :)

> Maybe next to set_cb kfunc add a comment that wq callbacks are sleepable.
> I think bpf prog writers are often with kernel background,
> so it will be their natural assumption that cb is sleepable.

I assume so as well.

Cheers,
Benjamin