drm/i915: Differentiate between aliasing-ppgtt and ggtt pinning

Userptr causes lockdep to complain when we are using the aliasing-ppgtt
(and ggtt, but for that it is rightfully so to complain about) in that
when we revoke the userptr we take a mutex which we also use to revoke
the mmaps. However, we only revoke mmaps for GGTT bindings and we never
allow userptr to create a GGTT binding so the warning should be false
and is simply caused by our conflation of the aliasing-ppgtt with the
ggtt. So lets try treating the binding into the aliasing-ppgtt as a
separate lockclass from the ggtt. The downside is that we are
deliberately suppressing lockdep;s ability to warn us of cycles.

Closes: https://gitlab.freedesktop.org/drm/intel/issues/478
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200326142727.31962-1-chris@chris-wilson.co.uk

1 files changed, 22 insertions, 3 deletions

diff --git a/drivers/gpu/drm/i915/i915_vma.c b/drivers/gpu/drm/i915/i915_vma.c
index 191577a98390..18069df2a9e5 100644
--- a/drivers/gpu/drm/i915/i915_vma.c
+++ b/drivers/gpu/drm/i915/i915_vma.c
@@ -913,11 +913,30 @@ int i915_vma_pin(struct i915_vma *vma, u64 size, u64 alignment, u64 flags)
 	if (flags & PIN_GLOBAL)
 		wakeref = intel_runtime_pm_get(&vma->vm->i915->runtime_pm);
 
-	/* No more allocations allowed once we hold vm->mutex */
-	err = mutex_lock_interruptible(&vma->vm->mutex);
+	/*
+	 * Differentiate between user/kernel vma inside the aliasing-ppgtt.
+	 *
+	 * We conflate the Global GTT with the user's vma when using the
+	 * aliasing-ppgtt, but it is still vitally important to try and
+	 * keep the use cases distinct. For example, userptr objects are
+	 * not allowed inside the Global GTT as that will cause lock
+	 * inversions when we have to evict them the mmu_notifier callbacks -
+	 * but they are allowed to be part of the user ppGTT which can never
+	 * be mapped. As such we try to give the distinct users of the same
+	 * mutex, distinct lockclasses [equivalent to how we keep i915_ggtt
+	 * and i915_ppgtt separate].
+	 *
+	 * NB this may cause us to mask real lock inversions -- while the
+	 * code is safe today, lockdep may not be able to spot future
+	 * transgressions.
+	 */
+	err = mutex_lock_interruptible_nested(&vma->vm->mutex,
+					      !(flags & PIN_GLOBAL));
 	if (err)
 		goto err_fence;
 
+	/* No more allocations allowed now we hold vm->mutex */
+
 	if (unlikely(i915_vma_is_closed(vma))) {
 		err = -ENOENT;
 		goto err_unlock;
@@ -1320,7 +1339,7 @@ int i915_vma_unbind(struct i915_vma *vma)
 	if (err)
 		goto out_rpm;
 
-	err = mutex_lock_interruptible(&vm->mutex);
+	err = mutex_lock_interruptible_nested(&vma->vm->mutex, !wakeref);
 	if (err)
 		goto out_rpm;