Re: [PATCH v3 1/2] drm/gpuvm: add deferred vm_bo cleanup

[Alice Ryhl <aliceryhl@google.com>]

On Wed, Oct 1, 2025 at 12:41 PM Alice Ryhl <aliceryhl@google.com> wrote:
>
> When using GPUVM in immediate mode, it is necessary to call
> drm_gpuvm_unlink() from the fence signalling critical path. However,
> unlink may call drm_gpuvm_bo_put(), which causes some challenges:
>
> 1. drm_gpuvm_bo_put() often requires you to take resv locks, which you
>    can't do from the fence signalling critical path.
> 2. drm_gpuvm_bo_put() calls drm_gem_object_put(), which is often going
>    to be unsafe to call from the fence signalling critical path.
>
> To solve these issues, add a deferred version of drm_gpuvm_unlink() that
> adds the vm_bo to a deferred cleanup list, and then clean it up later.
>
> The new methods take the GEMs GPUVA lock internally rather than letting
> the caller do it because it also needs to perform an operation after
> releasing the mutex again. This is to prevent freeing the GEM while
> holding the mutex (more info as comments in the patch). This means that
> the new methods can only be used with DRM_GPUVM_IMMEDIATE_MODE.
>
> Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>

In this version, I got rid of the kref_put_mutex() usage, but I
realized that maybe we should bring it back. With the current code,
it's actually possible to observe a zombie vm_bo in the GEM's list
because we don't drop the refcount while holding the mutex.

Alice