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

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Date:   Thu, 31 Aug 2023 12:56:58 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Christian König 
        <ckoenig.leichtzumerken@...il.com>
Cc:     Chia-I Wu <olvaffe@...il.com>, 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 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.