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

Jiri Slaby
2026-02-26T10:37:28Z

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

> 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...

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.

thanks,
--
js
suse labs