Re: [PATCH 1/4] drm/gpuvm: take GEM lock inside drm_gpuvm_bo_obtain_prealloc()

[Alice Ryhl <aliceryhl@google.com>]

On Fri, Nov 28, 2025 at 03:24:03PM +0100, Boris Brezillon wrote:
> On Fri, 28 Nov 2025 14:14:15 +0000
> Alice Ryhl <aliceryhl@google.com> wrote:
> 
> > When calling drm_gpuvm_bo_obtain_prealloc() and using immediate mode,
> > this may result in a call to ops->vm_bo_free(vm_bo) while holding the
> > GEMs gpuva mutex. This is a problem if ops->vm_bo_free(vm_bo) performs
> > any operations that are not safe in the fence signalling critical path,
> > and it turns out that Panthor (the only current user of the method)
> > calls drm_gem_shmem_unpin() which takes a resv lock internally.
> > 
> > This constitutes both a violation of signalling safety and lock
> > inversion. To fix this, we modify the method to internally take the GEMs
> > gpuva mutex so that the mutex can be unlocked before freeing the
> > preallocated vm_bo.
> > 
> > Note that this modification introduces a requirement that the driver
> > uses immediate mode to call drm_gpuvm_bo_obtain_prealloc() as it would
> > otherwise take the wrong lock.
> > 
> > Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> 
> Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
> 
> Should we add a Fixes tag?

Yeah, let's add:

Fixes: 63e919a31625 ("panthor: use drm_gpuva_unlink_defer()")

Alice