On 27/01/2022 11:56, Thomas Hellström wrote:
The vma destruction code was using an unlocked advisory check for
drm_mm_node_allocated() to avoid racing with eviction code unbinding
the vma.
This is very fragile and prohibits the dereference of non-refcounted
pointers of dying vmas after a call to __i915_vma_unbind(). It also
prohibits the dereference of vma->obj of refcounted pointers of
dying vmas after a call to __i915_vma_unbind(), since even if a
refcount is held on the vma, that won't guarantee that its backing
object doesn't get destroyed.
So introduce an unbind under the vm mutex at object destroy time,
removing all weak references of the vma and its object from the
object vma list and from the vm bound list.
Maarten suggested this fixes an oops like seen in
https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_22133/shard-snb6/igt@gem_softpin@xxxxxxxxxxxx.
If that is so, what would be the Fixes: tag to put here? Although it is
too late now so hopefully bug was introduced in something yet unreleased.
Signed-off-by: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
---
drivers/gpu/drm/i915/gem/i915_gem_object.c | 6 ++++++
1 file changed, 6 insertions(+)
diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object.c b/drivers/gpu/drm/i915/gem/i915_gem_object.c
index 1a9e1f940a7d..e03e362d320b 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_object.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_object.c
@@ -280,6 +280,12 @@ 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);
Hm I am not up to speed with the latest design, but how does the verb
verify and absence of conditionals reconcile here? Does the comment need
improving?
Regards,
Tvrtko
+
__i915_vma_put(vma);
spin_lock(&obj->vma.lock);
