[PATCH v2 12/16] drm/i915: Add i915_vma_unbind_unlocked, and take obj lock for i915_vma_unbind

[Matthew Auld]

On Mon, 29 Nov 2021 at 13:58, Maarten Lankhorst
<maarten.lankhorst at linux.intel.com> wrote:
>
> We want to remove more members of i915_vma, which requires the locking to be
> held more often.
>
> Start requiring gem object lock for i915_vma_unbind, as it's one of the
> callers that may unpin pages.
>
> Some special care is needed when evicting, because the last reference to the
> object may be held by the VMA, so after __i915_vma_unbind, vma may be garbage,
> and we need to cache vma->obj before unlocking.
>
> Signed-off-by: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> ---

<snip>

> @@ -129,22 +129,47 @@ void i915_ggtt_suspend_vm(struct i915_address_space *vm)
>
>         drm_WARN_ON(&vm->i915->drm, !vm->is_ggtt && !vm->is_dpt);
>
> +retry:
> +       i915_gem_drain_freed_objects(vm->i915);
> +
>         mutex_lock(&vm->mutex);
>
>         /* Skip rewriting PTE on VMA unbind. */
>         open = atomic_xchg(&vm->open, 0);
>
>         list_for_each_entry_safe(vma, vn, &vm->bound_list, vm_link) {
> +               struct drm_i915_gem_object *obj = vma->obj;
> +
>                 GEM_BUG_ON(!drm_mm_node_allocated(&vma->node));
> +
>                 i915_vma_wait_for_bind(vma);
>
> -               if (i915_vma_is_pinned(vma))
> +               if (i915_vma_is_pinned(vma) || !i915_vma_is_bound(vma, I915_VMA_GLOBAL_BIND))
>                         continue;
>
> -               if (!i915_vma_is_bound(vma, I915_VMA_GLOBAL_BIND)) {
> -                       __i915_vma_evict(vma);
> -                       drm_mm_remove_node(&vma->node);
> +               /* unlikely to race when GPU is idle, so no worry about slowpath.. */
> +               if (!i915_gem_object_trylock(obj, NULL)) {
> +                       atomic_set(&vm->open, open);

Does this need a comment about barriers?

> +
> +                       i915_gem_object_get(obj);

Should this not be kref_get_unless_zero? Assuming the vm->mutex is the
only thing keeping the object alive here, won't this lead to potential
uaf/double-free or something? Also should we not plonk this before the
trylock? Or maybe I'm missing something here?

> +                       mutex_unlock(&vm->mutex);
> +
> +                       i915_gem_object_lock(obj, NULL);
> +                       open = i915_vma_unbind(vma);
> +                       i915_gem_object_unlock(obj);
> +
> +                       GEM_WARN_ON(open);
> +
> +                       i915_gem_object_put(obj);
> +                       goto retry;
>                 }
> +
> +               i915_vma_wait_for_bind(vma);

We also call wait_for_bind above, is that intentional?