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Message-ID: <35c5a001-61b0-09d7-393c-b46836d0b676@canonical.com>
Date:   Thu, 5 May 2022 19:20:24 -0400
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Subject: Re: Issue With real-time patches on 5.15.y
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From:   Joseph Salisbury <joseph.salisbury@canonical.com>
To:     tglx@linutronix.de
Cc:     linux-kernel@vger.kernel.org, dave.hansen@linux.intel.com,
        linux-rt-users@vger.kernel.org
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On 5/5/22 19:18, Joseph Salisbury wrote:
> 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
Also, the following is the build failure that happens:
arch/x86/include/asm/pgtable.h:55:8: error: unknown type name 'spinlock_t'
    55 | extern spinlock_t pgd_lock;
       |        ^~~~~~~~~~