Re: Stalls when starting a VSOCK listening socket: soft lockups, RCU stalls, timeout

Jiri Slaby
2026-03-05T06:46:14Z

Hi,

On 03. 03. 26, 14:23, Matthieu Baerts wrote:
> On 26/02/2026 11:37, Jiri Slaby wrote:
>> On 06. 02. 26, 12:54, Matthieu Baerts wrote:
>>> Our CI for the MPTCP subsystem is now regularly hitting various stalls
>>> before even starting the MPTCP test suite. These issues are visible on
>>> top of the latest net and net-next trees, which have been sync with
>>> Linus' tree yesterday. All these issues have been seen on a "public CI"
>>> using GitHub-hosted runners with KVM support, where the tested kernel is
>>> launched in a nested (I suppose) VM. I can see the issue with or without
>>> debug.config. According to the logs, it might have started around
>>> v6.19-rc0, but I was unavailable for a few weeks, and I couldn't react
>>> quicker, sorry for that. Unfortunately, I cannot reproduce this locally,
>>> and the CI doesn't currently have the ability to execute bisections.
>>
>> Hmm, after the switch of the qemu guest kernels to 6.19, our (opensuse)
>> build service is stalling in smp_call_function_many_cond() randomly too:
>> https://bugzilla.suse.com/show_bug.cgi?id=1258936
>>
>> The attachment from there contains sysrq-t logs too:
>> https://bugzilla.suse.com/attachment.cgi?id=888612
>
> I'm glad I'm not the only one with this issue :)
>
> In your case, do you also have nested VMs with KVM support?

No, it's KVM directly on bare metal.

> Are you able to easily reproduce the issue and change the guest kernel
> in your build service?

Unfortunately no and no.

> On my side, any debugging steps need to be automated. Lately, it looks
> like the issue is more easily triggered on a stable 6.19 kernel, than on
> the last RC.
>
>>> The stalls happen before starting the MPTCP test suite. The init program
>>> creates a VSOCK listening socket via socat [1], and different hangs are
>>> then visible: RCU stalls followed by a soft lockup [2], only a soft
>>> lockup [3], sometimes the soft lockup comes with a delay [4] [5], or
>>> there is no RCU stalls or soft lockups detected after one minute, but VM
>>> is stalled [6]. In the last case, the VM is stopped after having
>>> launched GDB to get more details about what was being executed.
>>>
>>> It feels like the issue is not directly caused by the VSOCK listening
>>> socket, but the stalls always happen after having started the socat
>>> command [1] in the background.
>>
>> It fails randomly while building random packages (go, libreoffice,
>> bayle, ...). I don't think it is VSOCK related in those cases, but who
>> knows what the builds do...
>
> Indeed, unlikely to be VSOCK then.
>
>> I cannot reproduce locally either.
>>
>> I came across:
>>   614da1d3d4cd x86: make page fault handling disable interrupts properly
>> but I have no idea if it could have impact on this at all.
>
> Did it help to revert it?

We haven't tried, it is unlikely the cause.

--
js
suse labs