[Intel-gfx] [PATCH v2 1/3] drm/i915: Remove the vm open count

[Matthew Auld]

On Wed, 2 Mar 2022 at 10:22, Thomas Hellström
<thomas.hellstrom at linux.intel.com> wrote:
>
> vms are not getting properly closed. Rather than fixing that,
> Remove the vm open count and instead rely on the vm refcount.
>
> The vm open count existed solely to break the strong references the
> vmas had on the vms. Now instead make those references weak and
> ensure vmas are destroyed when the vm is destroyed.
>
> Unfortunately if the vm destructor and the object destructor both
> wants to destroy a vma, that may lead to a race in that the vm
> destructor just unbinds the vma and leaves the actual vma destruction
> to the object destructor. However in order for the object destructor
> to ensure the vma is unbound it needs to grab the vm mutex. In order
> to keep the vm mutex alive until the object destructor is done with
> it, somewhat hackishly grab a vm_resv refcount that is released late
> in the vma destruction process, when the vm mutex is no longer needed.
>
> v2: Address review-comments from Niranjana
> - Clarify that the struct i915_address_space::skip_pte_rewrite is a hack and
>   should ideally be replaced in an upcoming patch.
> - Remove an unneeded continue in clear_vm_list and update comment.
>
> Co-developed-by: Niranjana Vishwanathapura <niranjana.vishwanathapura at intel.com>
> Signed-off-by: Niranjana Vishwanathapura <niranjana.vishwanathapura at intel.com>
> Signed-off-by: Thomas Hellström <thomas.hellstrom at linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld at intel.com>