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From: Thomas Gleixner <tglx@kernel.org>
To: Jiri Slaby <jirislaby@kernel.org>, Matthieu Baerts <matttbe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>, Stefan Hajnoczi
 <stefanha@redhat.com>, Stefano Garzarella <sgarzare@redhat.com>,
 kvm@vger.kernel.org, virtualization@lists.linux.dev, Netdev
 <netdev@vger.kernel.org>, rcu@vger.kernel.org, MPTCP Linux
 <mptcp@lists.linux.dev>, Linux Kernel <linux-kernel@vger.kernel.org>,
 Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>, "Paul E. McKenney"
 <paulmck@kernel.org>, Dave Hansen <dave.hansen@linux.intel.com>,
 luto@kernel.org, Michal =?utf-8?Q?Koutn=C3=BD?= <MKoutny@suse.com>, Waiman
 Long
 <longman@redhat.com>, Marco Elver <elver@google.com>
Subject: Re: Stalls when starting a VSOCK listening socket: soft lockups,
 RCU stalls, timeout
In-Reply-To: <76e2b909-98db-49de-a8eb-f6f0a192f630@kernel.org>
References: <863a5291-a636-47d0-891c-bb0524d2e134@kernel.org>
 <20260302114636.GL606826@noisy.programming.kicks-ass.net>
 <717310d8-6274-4b7f-8a19-561c45f5f565@kernel.org>
 <a2b573b4-af61-4b84-a7d1-012ed6bb23c9@kernel.org>
 <ba067933-bf3b-476d-a0bb-53eda56996ca@kernel.org> <87zf4m2qvo.ffs@tglx>
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 <20260306152458.GT606826@noisy.programming.kicks-ass.net>
 <87ldg42eu7.ffs@tglx> <87h5qr2rzi.ffs@tglx> <87eclu3coa.ffs@tglx>
 <87v7f61cnl.ffs@tglx> <57c1e171-9520-4288-9e2d-10a72a499968@kernel.org>
 <87pl5ds88r.ffs@tglx> <0ae4d678-5676-4523-bae3-5ad73b526e27@kernel.org>
 <87eclsrtqg.ffs@tglx> <e846cb5f-65c4-4a9b-9f9b-593e4af526d8@kernel.org>
 <76e2b909-98db-49de-a8eb-f6f0a192f630@kernel.org>
Date: Tue, 10 Mar 2026 11:03:52 +0100
Message-ID: <878qc0rofr.ffs@tglx>
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On Tue, Mar 10 2026 at 10:00, Jiri Slaby wrote:
> On 10. 03. 26, 9:56, Jiri Slaby wrote:
>> Thomas,
>>=20
>> On 10. 03. 26, 9:09, Thomas Gleixner wrote:
>>> =C2=A0=C2=A0=C2=A0 2) task is not longer in the task list w/o going thr=
ough do_exit()
>>=20
>> After a quick look (I have to run now, so I could be completely wrong):=
=20
>> what about the task is not in the list *yet*. You use RCU ->=20
>> for_each_process_thread(), so there could be a race with attach_pid()=20
>> protected by tasklist_lock.
>
> Not attach_pid(), that's irrelevant here.
>
> But one line below in copy_process(): list_add_tail().

Yes. There is an issue. Peter and me just discovered that as well, but
in the case at hand it can't be the problem. The missing task is on the
CPU which means it must be visible in the thread list and task list.

Thanks,

        tglx