On Thu, Jun 24, 2021 at 04:03:13AM +0200, Jakub Jankowski wrote:

Hi,

We've recently hit this on 5.4.124: WARNING: CPU: 5 PID: 35 at kernel/kthread.c:75 kthread_is_per_cpu+0x21/0x30

After some digging, I figured out that this should be fixed by commit 3a7956e25e1d ("kthread: Fix PF_KTHREAD vs to_kthread() race") in mainline. It Fixes: ac687e6e8c26 ("kthread: Extract KTHREAD_IS_PER_CPU")

3a7956e25e1d has been backported to 5.12.x and 5.11.x:

• 5.12.4: as 163dd7fa459faebe055bcbebf9e088bae04f8e33
• 5.11.21: as 42e4caa93282ade6a143ff320d71062874dd3a5a

Makes sense, as ac687e6e8c26 originally landed in 5.11.

But it appears that since then ac687e6e8c26 has also been backported to LTS trees:

• 5.10.14: as b20475a80b4bd2c7bc720c3a9a8337c36b20dd8c
• 5.4.96: as 5b1e4fc2984eab30c5fffbf2bd1f016577cadab6
• 4.19.174: as fbad32181af9c8c76698b16115c28a493c26a2ee
• 4.14.220: as a9b2e4a94eee352e0b3e863045e7379d83131ee2

Maybe those trees could use a backport of 3a7956e25e1d as well then?

I would love to take a copy to those older trees, but the patch does not apply to those trees. Can you please provide a working backport that you have tested and we will be glad to take it.

thanks,

greg k-h