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Message-ID: <bdb23fe5-51ae-8873-a3d7-85f377877c79@canonical.com>
Date:   Thu, 5 May 2022 19:18:45 -0400
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Cc:     linux-kernel@vger.kernel.org, dave.hansen@linux.intel.com,
        linux-rt-users@vger.kernel.org
From:   Joseph Salisbury <joseph.salisbury@canonical.com>
To:     tglx@linutronix.de
Subject: Issue With real-time patches on 5.15.y
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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