Re: [PATCH 3/3] drm/amdgpu: wire up the can_remove() callback

[Daniel Vetter <daniel@ffwll.ch>]

On Tue, Feb 06, 2024 at 07:42:49PM +0100, Christian König wrote:
> 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.

Yeah it's exactly the same code, and removing the sysfs stuff means we
cant test hotunplug without physical hotunplugging stuff anymore. So
really not great - if one is buggy so is the other, and sysfs allows us to
control the timing a lot better to hit specific issues.
-Sima

> >   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
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch