linux-kernel - Re: [PATCH 3/3] drm/amdgpu: wire up the can

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Date: Tue, 6 Feb 2024 19:42:49 +0100
From: Christian König <ckoenig.leichtzumerken@...il.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Hamza Mahfooz <hamza.mahfooz@....com>, linux-kernel@...r.kernel.org,
 stable@...r.kernel.org, "Rafael J. Wysocki" <rafael@...nel.org>,
 Alex Deucher <alexander.deucher@....com>,
 Christian König <christian.koenig@....com>,
 "Pan, Xinhui" <Xinhui.Pan@....com>, David Airlie <airlied@...il.com>,
 Bjorn Helgaas <bhelgaas@...gle.com>,
 Mario Limonciello <mario.limonciello@....com>,
 Lijo Lazar <lijo.lazar@....com>,
 Srinivasan Shanmugam <srinivasan.shanmugam@....com>, Le Ma <le.ma@....com>,
 André Almeida <andrealmeid@...lia.com>,
 James Zhu <James.Zhu@....com>, Aurabindo Pillai <aurabindo.pillai@....com>,
 Alex Shi <alexs@...nel.org>, Jerry Snitselaar <jsnitsel@...hat.com>,
 Wei Liu <wei.liu@...nel.org>, Robin Murphy <robin.murphy@....com>,
 amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
 linux-pci@...r.kernel.org
Subject: Re: [PATCH 3/3] drm/amdgpu: wire up the can_remove() callback

Am 06.02.24 um 15:29 schrieb Daniel Vetter:
> On Fri, Feb 02, 2024 at 03:40:03PM -0800, Greg Kroah-Hartman wrote:
>> On Fri, Feb 02, 2024 at 05:25:56PM -0500, Hamza Mahfooz wrote:
>>> Removing an amdgpu device that still has user space references allocated
>>> to it causes undefined behaviour.
>> Then fix that please.  There should not be anything special about your
>> hardware that all of the tens of thousands of other devices can't handle
>> today.
>>
>> What happens when I yank your device out of a system with a pci hotplug
>> bus?  You can't prevent that either, so this should not be any different
>> at all.
>>
>> sorry, but please, just fix your driver.
> fwiw Christian König from amd already rejected this too, I have no idea
> why this was submitted

Well that was my fault.

I commented on an internal bug tracker that when sysfs bind/undbind is a 
different code path from PCI remove/re-scan we could try to reject it.

Turned out it isn't a different code path.

>   since the very elaborate plan I developed with a
> bunch of amd folks was to fix the various lifetime lolz we still have in
> drm. We unfortunately export the world of internal objects to userspace as
> uabi objects with dma_buf, dma_fence and everything else, but it's all
> fixable and we have the plan even documented:
>
> https://dri.freedesktop.org/docs/drm/gpu/drm-uapi.html#device-hot-unplug
>
> So yeah anything that isn't that plan of record is very much no-go for drm
> drivers. Unless we change that plan of course, but that needs a
> documentation patch first and a big discussion.
>
> Aside from an absolute massive pile of kernel-internal refcounting bugs
> the really big one we agreed on after a lot of discussion is that SIGBUS
> on dma-buf mmaps is no-go for drm drivers, because it would break way too
> much userspace in ways which are simply not fixable (since sig handlers
> are shared in a process, which means the gl/vk driver cannot use it).
>
> Otherwise it's bog standard "fix the kernel bugs" work, just a lot of it.

Ignoring a few memory leaks because of messed up refcounting we actually 
got that working quite nicely.

At least hot unplug / hot add seems to be working rather reliable in our 
internal testing.

So it can't be that messed up.

Regards,
Christian.

>
> Cheers, Sima