Why is "x86/split lock detection" spamming my syslog when Steam is running?

I have a ThinkPad X1C9 laptop connected to 3 displays via thunderbolt and an nVidia 1050Ti in an external GPU enclosure. My syslog is getting spammed with the following, why?

Googling CJobMgr led me to Steam. Closed Steam and it stopped. If I open Steam again it starts happening immediately.

[Tue Aug 10 10:17:42 2021] x86/split lock detection: #AC: CJobMgr::m_Work/28558 took a split_lock trap at address: 0xf21b3263
[Tue Aug 10 10:17:43 2021] x86/split lock detection: #AC: CJobMgr::m_Work/28520 took a split_lock trap at address: 0xf21b3263
[Tue Aug 10 10:17:43 2021] x86/split lock detection: #AC: CJobMgr::m_Work/28520 took a split_lock trap at address: 0xf21b3263
[Tue Aug 10 10:17:43 2021] x86/split lock detection: #AC: CJobMgr::m_Work/28558 took a split_lock trap at address: 0xf21b3263
[Tue Aug 10 10:17:44 2021] x86/split lock detection: #AC: CJobMgr::m_Work/28520 took a split_lock trap at address: 0xf21b3263
[Tue Aug 10 10:17:45 2021] x86/split lock detection: #AC: CJobMgr::m_Work/28558 took a split_lock trap at address: 0xf21b3263

alan@ac:~$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 21.04
Release:        21.04
Codename:       hirsute

alan@ac:~$ uname -a
Linux ac 5.11.0-25-generic #27-Ubuntu SMP Fri Jul 9 23:06:29 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

I have found the following resources during my search but they haven't helped:

• https://lwn.net/Articles/816918/
• https://lwn.net/Articles/790464/
• https://lwn.net/Articles/810379/

I have filed an issue upstream with the Steam client, as it's the only thing that triggers this issue:

• https://github.com/ValveSoftware/steam-for-linux/issues/8003

As you've found out, this is triggered by Steam which uses split locks despite them being discouraged by the kernel. There's a quite recent LWN.net article on this topic: https://lwn.net/Articles/911219/

As mentioned therein, since Linux 5.7 you can select the from following behaviors when a split lock is detected:

It gives the kernel the ability to respond to traps caused by split-lock operations and provides three modes for that response, selectable by the split_lock_detect= command-line parameter:

• off causes the kernel to behave as it did before; split-lock operations are not detected and nothing is done when they occur.
• warn (the default) causes a (rate-limited) warning to go to the system log when a split-lock operation is detected.
• fatal causes the kernel to immediately kill (with SIGBUS) any process attempting split-lock operations.

If you want to silence those warnings, set split_lock_detect=off as described in the Wiki. Meanwhile we wait for Valve to fix this.