linux-kernel - Re: [PATCH AUTOSEL 5.10 13/22] drm/amdgpu: install stub fence into potential unused fence pointers

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Date:   Mon, 4 Sep 2023 01:41:38 +0100
From:   Eddie Chapman <eddie@...k.net>
To:     Christian König <ckoenig.leichtzumerken@...il.com>,
        Chia-I Wu <olvaffe@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>
Cc:     Sasha Levin <sashal@...nel.org>, airlied@...ux.ie,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        amd-gfx@...ts.freedesktop.org,
        Alex Deucher <alexander.deucher@....com>,
        stable@...r.kernel.org, Lang Yu <Lang.Yu@....com>,
        Christian König <christian.koenig@....com>
Subject: Re: [PATCH AUTOSEL 5.10 13/22] drm/amdgpu: install stub fence into
 potential unused fence pointers

On 01/09/2023 07:02, Christian König wrote:
> Am 31.08.23 um 20:55 schrieb Chia-I Wu:
>> On Thu, Aug 31, 2023 at 7:01 AM Greg KH <gregkh@...uxfoundation.org> 
>> wrote:
>>> On Thu, Aug 31, 2023 at 03:26:28PM +0200, Christian König wrote:
>>>> Am 31.08.23 um 12:56 schrieb Greg KH:
>>>>> On Thu, Aug 31, 2023 at 12:27:27PM +0200, Christian König wrote:
>>>>>> Am 30.08.23 um 20:53 schrieb Chia-I Wu:
>>>>>>> On Sun, Jul 23, 2023 at 6:24 PM Sasha Levin <sashal@...nel.org> 
>>>>>>> wrote:
>>>>>>>> From: Lang Yu <Lang.Yu@....com>
>>>>>>>>
>>>>>>>> [ Upstream commit 187916e6ed9d0c3b3abc27429f7a5f8c936bd1f0 ]
>>>>>>>>
>>>>>>>> When using cpu to update page tables, vm update fences are unused.
>>>>>>>> Install stub fence into these fence pointers instead of NULL
>>>>>>>> to avoid NULL dereference when calling dma_fence_wait() on them.
>>>>>>>>
>>>>>>>> Suggested-by: Christian König <christian.koenig@....com>
>>>>>>>> Signed-off-by: Lang Yu <Lang.Yu@....com>
>>>>>>>> Reviewed-by: Christian König <christian.koenig@....com>
>>>>>>>> Signed-off-by: Alex Deucher <alexander.deucher@....com>
>>>>>>>> Signed-off-by: Sasha Levin <sashal@...nel.org>
>>>>>>>> ---
>>>>>>>>     drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c | 6 ++++--
>>>>>>>>     1 file changed, 4 insertions(+), 2 deletions(-)
>>>>>>> We start getting this warning spew on chromeos
>>>>>> Yeah because the older kernels still kept track of the last VM 
>>>>>> fence in the
>>>>>> syncobj.
>>>>>>
>>>>>> This patch here should probably not have been back ported.
>>>>>>
>>>>>> Why was that done anyway? The upstream commit doesn't have a CC 
>>>>>> stable and
>>>>>> this is only a bug fix for a new feature not present on older 
>>>>>> kernels.
>>>>> It is part of the AUTOSEL process.
>>>> Could we prevent patches from being backported by adding a Fixes: tag?
>>> Yes, that will show exactly where the patch should be backported to.
>> This is also AUTOSEL'ed to 5.15.  That might need a revert as well,
>> depending on when the amdgpu feature landed.
> 
> Both the feature and the bug fix landed in 6.5.
> 
> The bug fix should have never been backported to any older kernel at all 
> as far as I can see.
> 
> Regards,
> Christian.

I can confirm I've also started to get this warning hundreds of times 
per second having upgraded from 5.15.123 to 5.15.130. There's also a 
report from someone else of exactly the same on 5.15.128, here:
https://gitlab.freedesktop.org/drm/amd/-/issues/2820

Eddie