Re: [PATCH v2 2/3] drm/i915: Remove the vma refcount

[Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>]

On Wed, Mar 02, 2022 at 11:21:59AM +0100, Thomas Hellström wrote:
>Now that i915_vma_parked() is taking the object lock on vma destruction,
>and the only user of the vma refcount, i915_gem_object_unbind()
>also takes the object lock, remove the vma refcount.
>
>Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
>---
> drivers/gpu/drm/i915/i915_gem.c       | 17 +++++++++++++----
> drivers/gpu/drm/i915/i915_vma.c       | 14 +++-----------
> drivers/gpu/drm/i915/i915_vma.h       | 14 --------------
> drivers/gpu/drm/i915/i915_vma_types.h |  1 -
> 4 files changed, 16 insertions(+), 30 deletions(-)
>
>diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
>index dd84ebabb50f..c26110abcc0b 100644
>--- a/drivers/gpu/drm/i915/i915_gem.c
>+++ b/drivers/gpu/drm/i915/i915_gem.c
>@@ -151,14 +151,25 @@ int i915_gem_object_unbind(struct drm_i915_gem_object *obj,
> 			break;
> 		}
>
>+		/*
>+		 * Requiring the vm destructor to take the object lock
>+		 * before destroying a vma would help us eliminate the
>+		 * i915_vm_tryget() here, AND thus also the barrier stuff
>+		 * at the end. That's an easy fix, but sleeping locks in
>+		 * a kthread should generally be avoided.
>+		 */
> 		ret = -EAGAIN;
> 		if (!i915_vm_tryget(vma->vm))
> 			break;
>
>-		/* Prevent vma being freed by i915_vma_parked as we unbind */
>-		vma = __i915_vma_get(vma);
> 		spin_unlock(&obj->vma.lock);
>
>+		/*
>+		 * Since i915_vma_parked() takes the object lock
>+		 * before vma destruction, it won't race us here,
>+		 * and destroy the vma from under us.
>+		 */
>+
> 		if (vma) {
> 			bool vm_trylock = !!(flags & I915_GEM_OBJECT_UNBIND_VM_TRYLOCK);
> 			ret = -EBUSY;
>@@ -180,8 +191,6 @@ int i915_gem_object_unbind(struct drm_i915_gem_object *obj,
> 					ret = i915_vma_unbind(vma);
> 				}
> 			}
>-
>-			__i915_vma_put(vma);
> 		}
>
> 		i915_vm_put(vma->vm);

One issue still left in i915_gem_object_unbind is that it temporarily removes
vmas from the obj->vma.list and adds back later as vma needs to be unbind outside
the obj->vma.lock spinlock. This is an issue for other places where we iterate
over the obj->vma.list. i915_debugfs_describe_obj is one such case (upcoming
vm_bind will be another) that iterates over this list.
What is the plan here? Do we need to take object lock while iterating over the
list?

But this just something I noticed and not related to this patch.
This patch looks good to me.
Reviewed-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
 

>diff --git a/drivers/gpu/drm/i915/i915_vma.c b/drivers/gpu/drm/i915/i915_vma.c
>index 91538bc38110..6fd25b39748f 100644
>--- a/drivers/gpu/drm/i915/i915_vma.c
>+++ b/drivers/gpu/drm/i915/i915_vma.c
>@@ -122,7 +122,6 @@ vma_create(struct drm_i915_gem_object *obj,
> 	if (vma == NULL)
> 		return ERR_PTR(-ENOMEM);
>
>-	kref_init(&vma->ref);
> 	vma->ops = &vm->vma_ops;
> 	vma->obj = obj;
> 	vma->size = obj->base.size;
>@@ -1628,15 +1627,6 @@ void i915_vma_reopen(struct i915_vma *vma)
> 		__i915_vma_remove_closed(vma);
> }
>
>-void i915_vma_release(struct kref *ref)
>-{
>-	struct i915_vma *vma = container_of(ref, typeof(*vma), ref);
>-
>-	i915_active_fini(&vma->active);
>-	GEM_WARN_ON(vma->resource);
>-	i915_vma_free(vma);
>-}
>-
> static void force_unbind(struct i915_vma *vma)
> {
> 	if (!drm_mm_node_allocated(&vma->node))
>@@ -1665,7 +1655,9 @@ static void release_references(struct i915_vma *vma, bool vm_ddestroy)
> 	if (vm_ddestroy)
> 		i915_vm_resv_put(vma->vm);
>
>-	__i915_vma_put(vma);
>+	i915_active_fini(&vma->active);
>+	GEM_WARN_ON(vma->resource);
>+	i915_vma_free(vma);
> }
>
> /**
>diff --git a/drivers/gpu/drm/i915/i915_vma.h b/drivers/gpu/drm/i915/i915_vma.h
>index 67ae7341c7e0..6034991d89fe 100644
>--- a/drivers/gpu/drm/i915/i915_vma.h
>+++ b/drivers/gpu/drm/i915/i915_vma.h
>@@ -222,20 +222,6 @@ void i915_vma_unlink_ctx(struct i915_vma *vma);
> void i915_vma_close(struct i915_vma *vma);
> void i915_vma_reopen(struct i915_vma *vma);
>
>-static inline struct i915_vma *__i915_vma_get(struct i915_vma *vma)
>-{
>-	if (kref_get_unless_zero(&vma->ref))
>-		return vma;
>-
>-	return NULL;
>-}
>-
>-void i915_vma_release(struct kref *ref);
>-static inline void __i915_vma_put(struct i915_vma *vma)
>-{
>-	kref_put(&vma->ref, i915_vma_release);
>-}
>-
> void i915_vma_destroy_locked(struct i915_vma *vma);
> void i915_vma_destroy(struct i915_vma *vma);
>
>diff --git a/drivers/gpu/drm/i915/i915_vma_types.h b/drivers/gpu/drm/i915/i915_vma_types.h
>index eac36be184e5..be6e028c3b57 100644
>--- a/drivers/gpu/drm/i915/i915_vma_types.h
>+++ b/drivers/gpu/drm/i915/i915_vma_types.h
>@@ -211,7 +211,6 @@ struct i915_vma {
> 	 * handles (but same file) for execbuf, i.e. the number of aliases
> 	 * that exist in the ctx->handle_vmas LUT for this vma.
> 	 */
>-	struct kref ref;
> 	atomic_t open_count;
> 	atomic_t flags;
> 	/**
>-- 
>2.34.1
>