drm/panfrost: Fix a deadlock between the shrinker and madvise path

panfrost_ioctl_madvise() and panfrost_gem_purge() acquire the mappings
and shmem locks in different orders, thus leading to a potential
the mappings lock first.

Fixes: bdefca2d ("drm/panfrost: Add the panfrost_gem_mapping concept")
Cc: <stable@vger.kernel.org>
Cc: Christian Hewitt <christianshewitt@gmail.com>
Reported-by: Christian Hewitt <christianshewitt@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201101174016.839110-1-boris.brezillon@collabora.com

drivers/gpu/drm/panfrost/panfrost_gem.c

@@ -105,14 +105,12 @@ void panfrost_gem_mapping_put(struct panfrost_gem_mapping *mapping)
 	kref_put(&mapping->refcount, panfrost_gem_mapping_release);
 }
 
-void panfrost_gem_teardown_mappings(struct panfrost_gem_object *bo)
+void panfrost_gem_teardown_mappings_locked(struct panfrost_gem_object *bo)
 {
 	struct panfrost_gem_mapping *mapping;
 
-	mutex_lock(&bo->mappings.lock);
 	list_for_each_entry(mapping, &bo->mappings.list, node)
 		panfrost_gem_teardown_mapping(mapping);
-	mutex_unlock(&bo->mappings.lock);
 }
 
 int panfrost_gem_open(struct drm_gem_object *obj, struct drm_file *file_priv)

drivers/gpu/drm/panfrost/panfrost_gem.h

@@ -82,7 +82,7 @@ struct panfrost_gem_mapping *
 panfrost_gem_mapping_get(struct panfrost_gem_object *bo,
 			 struct panfrost_file_priv *priv);
 void panfrost_gem_mapping_put(struct panfrost_gem_mapping *mapping);
-void panfrost_gem_teardown_mappings(struct panfrost_gem_object *bo);
+void panfrost_gem_teardown_mappings_locked(struct panfrost_gem_object *bo);
 
 void panfrost_gem_shrinker_init(struct drm_device *dev);
 void panfrost_gem_shrinker_cleanup(struct drm_device *dev);

drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c

@@ -40,18 +40,26 @@ static bool panfrost_gem_purge(struct drm_gem_object *obj)
 {
 	struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
 	struct panfrost_gem_object *bo = to_panfrost_bo(obj);
+	bool ret = false;
 
 	if (atomic_read(&bo->gpu_usecount))
 		return false;
 
-	if (!mutex_trylock(&shmem->pages_lock))
+	if (!mutex_trylock(&bo->mappings.lock))
 		return false;
 
-	panfrost_gem_teardown_mappings(bo);
+	if (!mutex_trylock(&shmem->pages_lock))
+		goto unlock_mappings;
+
+	panfrost_gem_teardown_mappings_locked(bo);
 	drm_gem_shmem_purge_locked(obj);
+	ret = true;
 
 	mutex_unlock(&shmem->pages_lock);
-	return true;
+
+unlock_mappings:
+	mutex_unlock(&bo->mappings.lock);
+	return ret;
 }
 
 static unsigned long