[PATCH 1/6] drm/i915: Clarify vma lifetime
Thomas Hellström
thomas.hellstrom at linux.intel.com
Fri Feb 4 21:58:17 UTC 2022
It's unclear what reference the initial vma kref refernce refers to.
A vma can have multiple weak references, the object vma list,
the vm's bound list and the GT's closed_list, and the initial vma
reference can be put from lookups of all these lists.
With the current implementation this means
that any holder of yet another vma refcount (currently only
i915_gem_object_unbind()) needs to be holding two of either
*) An object refcount,
*) A vm open count
*) A vma open count
in order for us to not risk leaking a reference by having the
initial vma reference being put twice.
Address this by re-introducing i915_vma_destroy() which removes all
weak references of the vma and *then* puts the initial vma refcount.
This makes a strong vma reference hold on to the vma unconditionally.
Perhaps a better name would be i915_vma_revoke() or i915_vma_zombify(),
since other callers may still hold a refcount, but with the prospect of
being able to replace the vma refcount with the object lock in the near future,
let's stick with i915_vma_destroy().
Signed-off-by: Thomas Hellström <thomas.hellstrom at linux.intel.com>
---
drivers/gpu/drm/i915/gem/i915_gem_object.c | 14 +---
.../drm/i915/gem/selftests/i915_gem_mman.c | 4 +-
drivers/gpu/drm/i915/gt/intel_gtt.c | 17 +++--
drivers/gpu/drm/i915/i915_vma.c | 74 ++++++++++++++++---
drivers/gpu/drm/i915/i915_vma.h | 3 +
5 files changed, 79 insertions(+), 33 deletions(-)
diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object.c b/drivers/gpu/drm/i915/gem/i915_gem_object.c
index e03e362d320b..78c4cbe82031 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_object.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_object.c
@@ -267,12 +267,6 @@ void __i915_gem_object_pages_fini(struct drm_i915_gem_object *obj)
if (!list_empty(&obj->vma.list)) {
struct i915_vma *vma;
- /*
- * Note that the vma keeps an object reference while
- * it is active, so it *should* not sleep while we
- * destroy it. Our debug code errs insits it *might*.
- * For the moment, play along.
- */
spin_lock(&obj->vma.lock);
while ((vma = list_first_entry_or_null(&obj->vma.list,
struct i915_vma,
@@ -280,13 +274,7 @@ void __i915_gem_object_pages_fini(struct drm_i915_gem_object *obj)
GEM_BUG_ON(vma->obj != obj);
spin_unlock(&obj->vma.lock);
- /* Verify that the vma is unbound under the vm mutex. */
- mutex_lock(&vma->vm->mutex);
- atomic_and(~I915_VMA_PIN_MASK, &vma->flags);
- __i915_vma_unbind(vma);
- mutex_unlock(&vma->vm->mutex);
-
- __i915_vma_put(vma);
+ i915_vma_destroy(vma);
spin_lock(&obj->vma.lock);
}
diff --git a/drivers/gpu/drm/i915/gem/selftests/i915_gem_mman.c b/drivers/gpu/drm/i915/gem/selftests/i915_gem_mman.c
index ba29767348be..af36bffd064b 100644
--- a/drivers/gpu/drm/i915/gem/selftests/i915_gem_mman.c
+++ b/drivers/gpu/drm/i915/gem/selftests/i915_gem_mman.c
@@ -167,7 +167,7 @@ static int check_partial_mapping(struct drm_i915_gem_object *obj,
out:
i915_gem_object_lock(obj, NULL);
- __i915_vma_put(vma);
+ i915_vma_destroy(vma);
i915_gem_object_unlock(obj);
return err;
}
@@ -264,7 +264,7 @@ static int check_partial_mappings(struct drm_i915_gem_object *obj,
return err;
i915_gem_object_lock(obj, NULL);
- __i915_vma_put(vma);
+ i915_vma_destroy(vma);
i915_gem_object_unlock(obj);
if (igt_timeout(end_time,
diff --git a/drivers/gpu/drm/i915/gt/intel_gtt.c b/drivers/gpu/drm/i915/gt/intel_gtt.c
index 46be4197b93f..c9a24b021a7e 100644
--- a/drivers/gpu/drm/i915/gt/intel_gtt.c
+++ b/drivers/gpu/drm/i915/gt/intel_gtt.c
@@ -105,14 +105,19 @@ void __i915_vm_close(struct i915_address_space *vm)
list_for_each_entry_safe(vma, vn, &vm->bound_list, vm_link) {
struct drm_i915_gem_object *obj = vma->obj;
- /* Keep the obj (and hence the vma) alive as _we_ destroy it */
- if (!kref_get_unless_zero(&obj->base.refcount))
+ if (!kref_get_unless_zero(&obj->base.refcount)) {
+ /*
+ * Unbind the dying vma to ensure the bound_list
+ * is completely drained. We leave the destruction to
+ * the object destructor.
+ */
+ atomic_and(~I915_VMA_PIN_MASK, &vma->flags);
+ WARN_ON(__i915_vma_unbind(vma));
continue;
+ }
- atomic_and(~I915_VMA_PIN_MASK, &vma->flags);
- WARN_ON(__i915_vma_unbind(vma));
- __i915_vma_put(vma);
-
+ /* Keep the obj (and hence the vma) alive as _we_ destroy it */
+ i915_vma_destroy_locked(vma);
i915_gem_object_put(obj);
}
GEM_BUG_ON(!list_empty(&vm->bound_list));
diff --git a/drivers/gpu/drm/i915/i915_vma.c b/drivers/gpu/drm/i915/i915_vma.c
index 7815850b644e..9e1f7b224848 100644
--- a/drivers/gpu/drm/i915/i915_vma.c
+++ b/drivers/gpu/drm/i915/i915_vma.c
@@ -1606,15 +1606,27 @@ void i915_vma_reopen(struct i915_vma *vma)
void i915_vma_release(struct kref *ref)
{
struct i915_vma *vma = container_of(ref, typeof(*vma), ref);
+
+ i915_vm_put(vma->vm);
+ 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))
+ return;
+
+ atomic_and(~I915_VMA_PIN_MASK, &vma->flags);
+ WARN_ON(__i915_vma_unbind(vma));
+ GEM_BUG_ON(drm_mm_node_allocated(&vma->node));
+}
+
+static void release_references(struct i915_vma *vma)
+{
struct drm_i915_gem_object *obj = vma->obj;
- if (drm_mm_node_allocated(&vma->node)) {
- mutex_lock(&vma->vm->mutex);
- atomic_and(~I915_VMA_PIN_MASK, &vma->flags);
- WARN_ON(__i915_vma_unbind(vma));
- mutex_unlock(&vma->vm->mutex);
- GEM_BUG_ON(drm_mm_node_allocated(&vma->node));
- }
GEM_BUG_ON(i915_vma_is_active(vma));
spin_lock(&obj->vma.lock);
@@ -1624,11 +1636,49 @@ void i915_vma_release(struct kref *ref)
spin_unlock(&obj->vma.lock);
__i915_vma_remove_closed(vma);
- i915_vm_put(vma->vm);
- i915_active_fini(&vma->active);
- GEM_WARN_ON(vma->resource);
- i915_vma_free(vma);
+ __i915_vma_put(vma);
+}
+
+/**
+ * i915_vma_destroy_locked - Remove all weak reference to the vma and put
+ * the initial reference.
+ *
+ * This function should be called when it's decided the vma isn't needed
+ * anymore. The caller must assure that it doesn't race with another lookup
+ * plus destroy, typically by taking an appropriate reference.
+ *
+ * Current callsites are
+ * - __i915_gem_object_pages_fini()
+ * - __i915_vm_close() - Blocks the above function by taking a reference on
+ * the object.
+ * - __i915_vma_parked() - Blocks the above functions by taking an open-count on
+ * the vm and a reference on the object.
+ *
+ * Because of locks taken during destruction, a vma is also guaranteed to
+ * stay alive while the following locks are held if it was looked up while
+ * holding one of the locks:
+ * - vm->mutex
+ * - obj->vma.lock
+ * - gt->closed_lock
+ *
+ * A vma user can also temporarily keep the vma alive while holding a vma
+ * reference.
+ */
+void i915_vma_destroy_locked(struct i915_vma *vma)
+{
+ lockdep_assert_held(&vma->vm->mutex);
+
+ force_unbind(vma);
+ release_references(vma);
+}
+
+void i915_vma_destroy(struct i915_vma *vma)
+{
+ mutex_lock(&vma->vm->mutex);
+ force_unbind(vma);
+ mutex_unlock(&vma->vm->mutex);
+ release_references(vma);
}
void i915_vma_parked(struct intel_gt *gt)
@@ -1662,7 +1712,7 @@ void i915_vma_parked(struct intel_gt *gt)
if (i915_gem_object_trylock(obj, NULL)) {
INIT_LIST_HEAD(&vma->closed_link);
- __i915_vma_put(vma);
+ i915_vma_destroy(vma);
i915_gem_object_unlock(obj);
} else {
/* back you go.. */
diff --git a/drivers/gpu/drm/i915/i915_vma.h b/drivers/gpu/drm/i915/i915_vma.h
index 011af044ad4f..67ae7341c7e0 100644
--- a/drivers/gpu/drm/i915/i915_vma.h
+++ b/drivers/gpu/drm/i915/i915_vma.h
@@ -236,6 +236,9 @@ 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);
+
#define assert_vma_held(vma) dma_resv_assert_held((vma)->obj->base.resv)
static inline void i915_vma_lock(struct i915_vma *vma)
--
2.34.1
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