Issue With real-time patches on 5.15.y

[Joseph Salisbury <joseph.salisbury@canonical.com>]

Hello,

I ran into a build failure after applying the rt40 real-time patches[0] 
to an Ubuntu 22.04 kernel. The Ubuntu kernel is based on upstream stable 
5.15.y(5.15.34 currently).  I believe this patch is the specific one 
that introduces the build issue: 
0086-locking-Allow-to-include-asm-spinlock_types.h-from-l.patch.

After a bisect, the following commit was identified as the cause for the 
build failure:
b50854eca0e0 ("x86/pkru: Remove useless include")

This commit was added to mainline as of 5.16-rc1 and not Cc'd to stable, 
so it should not exist in 5.15.y.  However, it was purposely added to 
the Ubuntu kernel to enable x86 AMX support[1].

The real-time kernel build failure can be resolved by reverting commit 
b50854eca0e0.  The failure seems to be due to the removal of an include 
of xstate.h from pkru.h and caused spinlock_t to not be defined.  The 
commit would only be reverted for the real-time kernel and not any other 
kernels.  I wanted to see if reverting the commit is the proper 
approach, or if cherry-picking additional commits might be a better 
solution in preparation for additional changes that might be coming in 
the future?

Any suggestions would be greatly appreciated.

Thanks,

Joe


[0] 
https://cdn.kernel.org/pub/linux/kernel/projects/rt/5.15/older/patches-5.15.34-rt40.tar.gz
[1] https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1967750