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From: Jakub Sitnicki <jakub@cloudflare.com>
To: Hillf Danton <hdanton@sina.com>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>,  Tetsuo Handa
 <penguin-kernel@i-love.sakura.ne.jp>,  Eric Dumazet <edumazet@google.com>,
  Linus Torvalds <torvalds@linux-foundation.org>,  bpf
 <bpf@vger.kernel.org>,  LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] bpf, sockmap: defer sk_psock_free_link() using RCU
In-Reply-To: <20240521225918.2147-1-hdanton@sina.com> (Hillf Danton's message
	of "Wed, 22 May 2024 06:59:18 +0800")
References: <838e7959-a360-4ac1-b36a-a3469236129b@I-love.SAKURA.ne.jp>
	<20240521225918.2147-1-hdanton@sina.com>
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Date: Wed, 22 May 2024 11:50:49 +0200
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On Wed, May 22, 2024 at 06:59 AM +08, Hillf Danton wrote:
> On Tue, 21 May 2024 08:38:52 -0700 Alexei Starovoitov <alexei.starovoitov@gmail.com>
>> On Sun, May 12, 2024 at 12:22=E2=80=AFAM Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp> wrote:
>> > --- a/net/core/sock_map.c
>> > +++ b/net/core/sock_map.c
>> > @@ -142,6 +142,7 @@ static void sock_map_del_link(struct sock *sk,
>> >         bool strp_stop =3D false, verdict_stop =3D false;
>> >         struct sk_psock_link *link, *tmp;
>> >
>> > +       rcu_read_lock();
>> >         spin_lock_bh(&psock->link_lock);
>> 
>> I think this is incorrect.
>> spin_lock_bh may sleep in RT and it won't be safe to do in rcu cs.
>
> Could you specify why it won't be safe in rcu cs if you are right?
> What does rcu look like in RT if not nothing?

RCU readers can't block, while spinlock RT doesn't disable preemption.

https://docs.kernel.org/RCU/rcu.html
https://docs.kernel.org/locking/locktypes.html#spinlock-t-and-preempt-rt

I've finally gotten around to testing proposed fix that just disallows
map_delete_elem on sockmap/sockhash from BPF tracing progs
completely. This should put an end to this saga of syzkaller reports.

https://lore.kernel.org/all/87jzjnxaqf.fsf@cloudflare.com/