drm/i915: Untangle the vma pages_mutex

Any sleeping dma_resv lock taken while the vma pages_mutex is held
will cause a lockdep splat.
Move the i915_gem_object_pin_pages() call out of the pages_mutex
critical section.
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210601074654.3103-2-thomas.hellstrom@linux.intel.com

drivers/gpu/drm/i915/i915_vma.c

@@ -800,32 +800,37 @@ static bool try_qad_pin(struct i915_vma *vma, unsigned int flags)
 static int vma_get_pages(struct i915_vma *vma)
 {
 	int err = 0;
+	bool pinned_pages = false;
 
 	if (atomic_add_unless(&vma->pages_count, 1, 0))
 		return 0;
 
+	if (vma->obj) {
+		err = i915_gem_object_pin_pages(vma->obj);
+		if (err)
+			return err;
+		pinned_pages = true;
+	}
+
 	/* Allocations ahoy! */
-	if (mutex_lock_interruptible(&vma->pages_mutex))
-		return -EINTR;
+	if (mutex_lock_interruptible(&vma->pages_mutex)) {
+		err = -EINTR;
+		goto unpin;
+	}
 
 	if (!atomic_read(&vma->pages_count)) {
-		if (vma->obj) {
-			err = i915_gem_object_pin_pages(vma->obj);
-			if (err)
-				goto unlock;
-		}
-
 		err = vma->ops->set_pages(vma);
-		if (err) {
-			if (vma->obj)
-				i915_gem_object_unpin_pages(vma->obj);
+		if (err)
 			goto unlock;
-		}
+		pinned_pages = false;
 	}
 	atomic_inc(&vma->pages_count);
 
 unlock:
 	mutex_unlock(&vma->pages_mutex);
+unpin:
+	if (pinned_pages)
+		__i915_gem_object_unpin_pages(vma->obj);
 
 	return err;
 }