Re: [PATCH v8 5/7] drm: avoid circular locks in drm_mode_object_find

From: Daniel Vetter
Date: Thu Aug 26 2021 - 09:13:34 EST

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In reply to: Desmond Cheong Zhi Xi: "[PATCH v8 5/7] drm: avoid circular locks in drm_mode_object_find"
Next in thread: Desmond Cheong Zhi Xi: "[PATCH v8 6/7] drm: avoid circular locks in drm_lease_held"
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On Thu, Aug 26, 2021 at 10:01:20AM +0800, Desmond Cheong Zhi Xi wrote:
> __drm_mode_object_find checks if the given drm file holds the required
> lease on a object by calling _drm_lease_held. _drm_lease_held in turn
> uses drm_file_get_master to access drm_file.master.
>
> However, in a future patch, the drm_file.master_lookup_lock in
> drm_file_get_master will be replaced by drm_device.master_rwsem. This
> is an issue for two reasons:
>
> 1. master_rwsem is sometimes already held when __drm_mode_object_find
> is called, which leads to recursive locks on master_rwsem
>
> 2. drm_mode_object_find is sometimes called with the modeset_mutex
> held, which leads to an inversion of the master_rwsem -->
> modeset_mutex lock hierarchy
>
> To fix this, we make __drm_mode_object_find the locked version of
> drm_mode_object_find, and wrap calls to __drm_mode_object_find with
> locks on master_rwsem. This allows us to safely access drm_file.master
> in _drm_lease_held (__drm_mode_object_find is its only caller) without
> the use of drm_file_get_master.
>
> Functions that already lock master_rwsem are modified to call
> __drm_mode_object_find, whereas functions that haven't locked
> master_rwsem should call drm_mode_object_find. These two options
> allow us to grab master_rwsem before modeset_mutex (such as in
> drm_mode_get_obj_get_properties_ioctl).
>
> This new rule requires more extensive changes to three functions:
> drn_connector_lookup, drm_crtc_find, and drm_plane_find. These
> functions are only sometimes called with master_rwsem held. Hence, we
> have to further split them into locked and unlocked versions that call
> __drm_mode_object_find and drm_mode_object_find respectively.

I think approach looks good, but the naming isn't so great. Usually __
prefix means "do not call directly, this is only exported for static
inline and other helpers". For these the usual rule is to add a _locked or
_unlocked suffix. I'd leave the normal _find functions as-is (since those
take the lock) themselves, and annotate the _locked ones.

Also same for the other lookup helpers.

>
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>
> ---
> drivers/gpu/drm/drm_atomic_uapi.c | 7 ++---
> drivers/gpu/drm/drm_color_mgmt.c | 2 +-
> drivers/gpu/drm/drm_crtc.c | 5 ++--
> drivers/gpu/drm/drm_framebuffer.c | 2 +-
> drivers/gpu/drm/drm_lease.c | 21 +++++----------
> drivers/gpu/drm/drm_mode_object.c | 28 +++++++++++++++++---
> drivers/gpu/drm/drm_plane.c | 8 +++---
> drivers/gpu/drm/drm_property.c | 6 ++---
> drivers/gpu/drm/i915/display/intel_overlay.c | 2 +-
> drivers/gpu/drm/i915/display/intel_sprite.c | 2 +-
> drivers/gpu/drm/vmwgfx/vmwgfx_kms.c | 2 +-
> include/drm/drm_connector.h | 23 ++++++++++++++++
> include/drm/drm_crtc.h | 22 +++++++++++++++
> include/drm/drm_mode_object.h | 3 +++
> include/drm/drm_plane.h | 20 ++++++++++++++
> 15 files changed, 118 insertions(+), 35 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_atomic_uapi.c b/drivers/gpu/drm/drm_atomic_uapi.c
> index 909f31833181..cda9a501cf74 100644
> --- a/drivers/gpu/drm/drm_atomic_uapi.c
> +++ b/drivers/gpu/drm/drm_atomic_uapi.c
> @@ -557,7 +557,7 @@ static int drm_atomic_plane_set_property(struct drm_plane *plane,
> return -EINVAL;
>
> } else if (property == config->prop_crtc_id) {
> - struct drm_crtc *crtc = drm_crtc_find(dev, file_priv, val);
> + struct drm_crtc *crtc = __drm_crtc_find(dev, file_priv, val);
>
> if (val && !crtc)
> return -EACCES;
> @@ -709,7 +709,7 @@ static int drm_atomic_connector_set_property(struct drm_connector *connector,
> int ret;
>
> if (property == config->prop_crtc_id) {
> - struct drm_crtc *crtc = drm_crtc_find(dev, file_priv, val);
> + struct drm_crtc *crtc = __drm_crtc_find(dev, file_priv, val);
>
> if (val && !crtc)
> return -EACCES;
> @@ -1385,7 +1385,8 @@ int drm_mode_atomic_ioctl(struct drm_device *dev,
> goto out;
> }
>
> - obj = drm_mode_object_find(dev, file_priv, obj_id, DRM_MODE_OBJECT_ANY);
> + obj = __drm_mode_object_find(dev, file_priv, obj_id,
> + DRM_MODE_OBJECT_ANY);
> if (!obj) {
> ret = -ENOENT;
> goto out;
> diff --git a/drivers/gpu/drm/drm_color_mgmt.c b/drivers/gpu/drm/drm_color_mgmt.c
> index bb14f488c8f6..9dcb2ccca3ab 100644
> --- a/drivers/gpu/drm/drm_color_mgmt.c
> +++ b/drivers/gpu/drm/drm_color_mgmt.c
> @@ -365,7 +365,7 @@ int drm_mode_gamma_set_ioctl(struct drm_device *dev,
> if (!drm_core_check_feature(dev, DRIVER_MODESET))
> return -EOPNOTSUPP;
>
> - crtc = drm_crtc_find(dev, file_priv, crtc_lut->crtc_id);
> + crtc = __drm_crtc_find(dev, file_priv, crtc_lut->crtc_id);
> if (!crtc)
> return -ENOENT;
>
> diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
> index 26a77a735905..b1279bb3fa61 100644
> --- a/drivers/gpu/drm/drm_crtc.c
> +++ b/drivers/gpu/drm/drm_crtc.c
> @@ -656,7 +656,7 @@ int drm_mode_setcrtc(struct drm_device *dev, void *data,
> if (crtc_req->x & 0xffff0000 || crtc_req->y & 0xffff0000)
> return -ERANGE;
>
> - crtc = drm_crtc_find(dev, file_priv, crtc_req->crtc_id);
> + crtc = __drm_crtc_find(dev, file_priv, crtc_req->crtc_id);
> if (!crtc) {
> DRM_DEBUG_KMS("Unknown CRTC ID %d\n", crtc_req->crtc_id);
> return -ENOENT;
> @@ -787,7 +787,8 @@ int drm_mode_setcrtc(struct drm_device *dev, void *data,
> goto out;
> }
>
> - connector = drm_connector_lookup(dev, file_priv, out_id);
> + connector = __drm_connector_lookup(dev, file_priv,
> + out_id);
> if (!connector) {
> DRM_DEBUG_KMS("Connector id %d unknown\n",
> out_id);
> diff --git a/drivers/gpu/drm/drm_framebuffer.c b/drivers/gpu/drm/drm_framebuffer.c
> index 07f5abc875e9..9c1db91b150d 100644
> --- a/drivers/gpu/drm/drm_framebuffer.c
> +++ b/drivers/gpu/drm/drm_framebuffer.c
> @@ -887,7 +887,7 @@ struct drm_framebuffer *drm_framebuffer_lookup(struct drm_device *dev,
> struct drm_mode_object *obj;
> struct drm_framebuffer *fb = NULL;
>
> - obj = __drm_mode_object_find(dev, file_priv, id, DRM_MODE_OBJECT_FB);
> + obj = drm_mode_object_find(dev, file_priv, id, DRM_MODE_OBJECT_FB);

I expect this to go boom, also for property and blob objects. These lookup
functions can be called from the atomic_ioctl machinery, where we're
already holding all kinds of locks. So grabbing the master_rwsem is not a
good idea.

We should always use the the drm_mode_object_find_unlocked for anything
object which is not in the list of drm_mode_object_lease_required().

> if (obj)
> fb = obj_to_fb(obj);
> return fb;
> diff --git a/drivers/gpu/drm/drm_lease.c b/drivers/gpu/drm/drm_lease.c
> index bed6f7636cbe..1b156c85d1c8 100644
> --- a/drivers/gpu/drm/drm_lease.c
> +++ b/drivers/gpu/drm/drm_lease.c
> @@ -105,22 +105,13 @@ static bool _drm_has_leased(struct drm_master *master, int id)
> return false;
> }
>
> -/* Called with idr_mutex held */
> +/* Called with idr_mutex and master_rwsem held */

Please verify this with lockdep_assert_held.

The reason is that our locking gets rather funny here, because only for
some types of objects do we need to check master status.
drm_mode_object_lease_required() decides that.

> bool _drm_lease_held(struct drm_file *file_priv, int id)
> {
> - bool ret;
> - struct drm_master *master;
> -
> - if (!file_priv)
> + if (!file_priv || !file_priv->master)
> return true;
>
> - master = drm_file_get_master(file_priv);
> - if (!master)
> - return true;
> - ret = _drm_lease_held_master(master, id);
> - drm_master_put(&master);
> -
> - return ret;
> + return _drm_lease_held_master(file_priv->master, id);
> }
>
> bool drm_lease_held(struct drm_file *file_priv, int id)
> @@ -391,9 +382,9 @@ static int fill_object_idr(struct drm_device *dev,
> /* step one - get references to all the mode objects
> and check for validity. */
> for (o = 0; o < object_count; o++) {
> - objects[o] = drm_mode_object_find(dev, lessor_priv,
> - object_ids[o],
> - DRM_MODE_OBJECT_ANY);
> + objects[o] = __drm_mode_object_find(dev, lessor_priv,
> + object_ids[o],
> + DRM_MODE_OBJECT_ANY);
> if (!objects[o]) {
> ret = -ENOENT;
> goto out_free_objects;
> diff --git a/drivers/gpu/drm/drm_mode_object.c b/drivers/gpu/drm/drm_mode_object.c
> index 86d9e907c0b2..90c23997aa53 100644
> --- a/drivers/gpu/drm/drm_mode_object.c
> +++ b/drivers/gpu/drm/drm_mode_object.c
> @@ -133,12 +133,27 @@ bool drm_mode_object_lease_required(uint32_t type)
> }
> }
>
> +/**
> + * __drm_mode_object_find - look up a drm object with static lifetime
> + * @dev: drm device
> + * @file_priv: drm file
> + * @id: id of the mode object
> + * @type: type of the mode object
> + *
> + * This function is used to look up a modeset object. It will acquire a
> + * reference for reference counted objects. This reference must be dropped
> + * again by calling drm_mode_object_put().
> + *
> + * Similar to drm_mode_object_find(), but called with &drm_device.master_rwsem
> + * held.
> + */
> struct drm_mode_object *__drm_mode_object_find(struct drm_device *dev,
> struct drm_file *file_priv,
> - uint32_t id, uint32_t type)
> + u32 id, u32 type)
> {
> struct drm_mode_object *obj = NULL;
>
> + lockdep_assert_held_once(&dev->master_rwsem);
> mutex_lock(&dev->mode_config.idr_mutex);
> obj = idr_find(&dev->mode_config.object_idr, id);
> if (obj && type != DRM_MODE_OBJECT_ANY && obj->type != type)
> @@ -158,6 +173,7 @@ struct drm_mode_object *__drm_mode_object_find(struct drm_device *dev,
>
> return obj;
> }
> +EXPORT_SYMBOL(__drm_mode_object_find);
>
> /**
> * drm_mode_object_find - look up a drm object with static lifetime
> @@ -176,7 +192,9 @@ struct drm_mode_object *drm_mode_object_find(struct drm_device *dev,
> {
> struct drm_mode_object *obj = NULL;
>
> + down_read(&dev->master_rwsem);
> obj = __drm_mode_object_find(dev, file_priv, id, type);
> + up_read(&dev->master_rwsem);
> return obj;
> }
> EXPORT_SYMBOL(drm_mode_object_find);
> @@ -408,9 +426,12 @@ int drm_mode_obj_get_properties_ioctl(struct drm_device *dev, void *data,
> if (!drm_core_check_feature(dev, DRIVER_MODESET))
> return -EOPNOTSUPP;
>
> + down_read(&dev->master_rwsem);
> DRM_MODESET_LOCK_ALL_BEGIN(dev, ctx, 0, ret);
>
> - obj = drm_mode_object_find(dev, file_priv, arg->obj_id, arg->obj_type);
> + obj = __drm_mode_object_find(dev, file_priv, arg->obj_id,
> + arg->obj_type);
> + up_read(&dev->master_rwsem);
> if (!obj) {
> ret = -ENOENT;
> goto out;
> @@ -534,7 +555,8 @@ int drm_mode_obj_set_property_ioctl(struct drm_device *dev, void *data,
> if (!drm_core_check_feature(dev, DRIVER_MODESET))
> return -EOPNOTSUPP;
>
> - arg_obj = drm_mode_object_find(dev, file_priv, arg->obj_id, arg->obj_type);
> + arg_obj = __drm_mode_object_find(dev, file_priv, arg->obj_id,
> + arg->obj_type);
> if (!arg_obj)
> return -ENOENT;
>
> diff --git a/drivers/gpu/drm/drm_plane.c b/drivers/gpu/drm/drm_plane.c
> index 82afb854141b..b5566167a798 100644
> --- a/drivers/gpu/drm/drm_plane.c
> +++ b/drivers/gpu/drm/drm_plane.c
> @@ -971,7 +971,7 @@ int drm_mode_setplane(struct drm_device *dev, void *data,
> * First, find the plane, crtc, and fb objects. If not available,
> * we don't bother to call the driver.
> */
> - plane = drm_plane_find(dev, file_priv, plane_req->plane_id);
> + plane = __drm_plane_find(dev, file_priv, plane_req->plane_id);
> if (!plane) {
> DRM_DEBUG_KMS("Unknown plane ID %d\n",
> plane_req->plane_id);
> @@ -986,7 +986,7 @@ int drm_mode_setplane(struct drm_device *dev, void *data,
> return -ENOENT;
> }
>
> - crtc = drm_crtc_find(dev, file_priv, plane_req->crtc_id);
> + crtc = __drm_crtc_find(dev, file_priv, plane_req->crtc_id);
> if (!crtc) {
> drm_framebuffer_put(fb);
> DRM_DEBUG_KMS("Unknown crtc ID %d\n",
> @@ -1108,7 +1108,7 @@ static int drm_mode_cursor_common(struct drm_device *dev,
> if (!req->flags || (~DRM_MODE_CURSOR_FLAGS & req->flags))
> return -EINVAL;
>
> - crtc = drm_crtc_find(dev, file_priv, req->crtc_id);
> + crtc = __drm_crtc_find(dev, file_priv, req->crtc_id);
> if (!crtc) {
> DRM_DEBUG_KMS("Unknown CRTC ID %d\n", req->crtc_id);
> return -ENOENT;
> @@ -1229,7 +1229,7 @@ int drm_mode_page_flip_ioctl(struct drm_device *dev,
> if ((page_flip->flags & DRM_MODE_PAGE_FLIP_ASYNC) && !dev->mode_config.async_page_flip)
> return -EINVAL;
>
> - crtc = drm_crtc_find(dev, file_priv, page_flip->crtc_id);
> + crtc = __drm_crtc_find(dev, file_priv, page_flip->crtc_id);
> if (!crtc)
> return -ENOENT;
>
> diff --git a/drivers/gpu/drm/drm_property.c b/drivers/gpu/drm/drm_property.c
> index 6c353c9dc772..9f04dcb81d07 100644
> --- a/drivers/gpu/drm/drm_property.c
> +++ b/drivers/gpu/drm/drm_property.c
> @@ -656,7 +656,7 @@ struct drm_property_blob *drm_property_lookup_blob(struct drm_device *dev,
> struct drm_mode_object *obj;
> struct drm_property_blob *blob = NULL;
>
> - obj = __drm_mode_object_find(dev, NULL, id, DRM_MODE_OBJECT_BLOB);
> + obj = drm_mode_object_find(dev, NULL, id, DRM_MODE_OBJECT_BLOB);
> if (obj)
> blob = obj_to_blob(obj);
> return blob;
> @@ -919,8 +919,8 @@ bool drm_property_change_valid_get(struct drm_property *property,
> if (value == 0)
> return true;
>
> - *ref = __drm_mode_object_find(property->dev, NULL, value,
> - property->values[0]);
> + *ref = drm_mode_object_find(property->dev, NULL, value,
> + property->values[0]);
> return *ref != NULL;
> }
>
> diff --git a/drivers/gpu/drm/i915/display/intel_overlay.c b/drivers/gpu/drm/i915/display/intel_overlay.c
> index 7e3f5c6ca484..1d4af29e31c9 100644
> --- a/drivers/gpu/drm/i915/display/intel_overlay.c
> +++ b/drivers/gpu/drm/i915/display/intel_overlay.c
> @@ -1120,7 +1120,7 @@ int intel_overlay_put_image_ioctl(struct drm_device *dev, void *data,
> return ret;
> }
>
> - drmmode_crtc = drm_crtc_find(dev, file_priv, params->crtc_id);
> + drmmode_crtc = __drm_crtc_find(dev, file_priv, params->crtc_id);
> if (!drmmode_crtc)
> return -ENOENT;
> crtc = to_intel_crtc(drmmode_crtc);
> diff --git a/drivers/gpu/drm/i915/display/intel_sprite.c b/drivers/gpu/drm/i915/display/intel_sprite.c
> index 4ae9a7455b23..e19ef2d90bac 100644
> --- a/drivers/gpu/drm/i915/display/intel_sprite.c
> +++ b/drivers/gpu/drm/i915/display/intel_sprite.c
> @@ -1505,7 +1505,7 @@ int intel_sprite_set_colorkey_ioctl(struct drm_device *dev, void *data,
> set->flags & I915_SET_COLORKEY_DESTINATION)
> return -EINVAL;
>
> - plane = drm_plane_find(dev, file_priv, set->plane_id);
> + plane = __drm_plane_find(dev, file_priv, set->plane_id);
> if (!plane || plane->type != DRM_PLANE_TYPE_OVERLAY)
> return -ENOENT;
>
> diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_kms.c b/drivers/gpu/drm/vmwgfx/vmwgfx_kms.c
> index 74fa41909213..d368b2bcb1fa 100644
> --- a/drivers/gpu/drm/vmwgfx/vmwgfx_kms.c
> +++ b/drivers/gpu/drm/vmwgfx/vmwgfx_kms.c
> @@ -1862,7 +1862,7 @@ int vmw_kms_cursor_bypass_ioctl(struct drm_device *dev, void *data,
> return 0;
> }
>
> - crtc = drm_crtc_find(dev, file_priv, arg->crtc_id);
> + crtc = __drm_crtc_find(dev, file_priv, arg->crtc_id);
> if (!crtc) {
> ret = -ENOENT;
> goto out;
> diff --git a/include/drm/drm_connector.h b/include/drm/drm_connector.h
> index 1647960c9e50..322c823583c7 100644
> --- a/include/drm/drm_connector.h
> +++ b/include/drm/drm_connector.h
> @@ -1591,6 +1591,29 @@ static inline u32 drm_connector_mask(const struct drm_connector *connector)
> return 1 << connector->index;
> }
>
> +/**
> + * __drm_connector_lookup - lookup connector object
> + * @dev: DRM device
> + * @file_priv: drm file to check for lease against.
> + * @id: connector object id
> + *
> + * This function looks up the connector object specified by id
> + * add takes a reference to it.
> + *
> + * Similar to drm_connector_lookup(), but called with &drm_device.master_rwsem
> + * held.
> + */
> +static inline struct drm_connector *__drm_connector_lookup(struct drm_device *dev,
> + struct drm_file *file_priv,
> + uint32_t id)
> +{
> + struct drm_mode_object *mo;
> +
> + mo = __drm_mode_object_find(dev, file_priv, id,
> + DRM_MODE_OBJECT_CONNECTOR);
> + return mo ? obj_to_connector(mo) : NULL;
> +}
> +
> /**
> * drm_connector_lookup - lookup connector object
> * @dev: DRM device
> diff --git a/include/drm/drm_crtc.h b/include/drm/drm_crtc.h
> index 13eeba2a750a..69df854dd322 100644
> --- a/include/drm/drm_crtc.h
> +++ b/include/drm/drm_crtc.h
> @@ -1283,6 +1283,28 @@ static inline uint32_t drm_crtc_mask(const struct drm_crtc *crtc)
> int drm_mode_set_config_internal(struct drm_mode_set *set);
> struct drm_crtc *drm_crtc_from_index(struct drm_device *dev, int idx);
>
> +/**
> + * __drm_crtc_find - look up a CRTC object from its ID
> + * @dev: DRM device
> + * @file_priv: drm file to check for lease against.
> + * @id: &drm_mode_object ID
> + *
> + * This can be used to look up a CRTC from its userspace ID. Only used by
> + * drivers for legacy IOCTLs and interface, nowadays extensions to the KMS
> + * userspace interface should be done using &drm_property.
> + *
> + * Similar to drm_crtc_find(), but called with &drm_device.master_rwsem held.
> + */
> +static inline struct drm_crtc *__drm_crtc_find(struct drm_device *dev,
> + struct drm_file *file_priv,
> + uint32_t id)
> +{
> + struct drm_mode_object *mo;
> +
> + mo = __drm_mode_object_find(dev, file_priv, id, DRM_MODE_OBJECT_CRTC);
> + return mo ? obj_to_crtc(mo) : NULL;
> +}
> +
> /**
> * drm_crtc_find - look up a CRTC object from its ID
> * @dev: DRM device
> diff --git a/include/drm/drm_mode_object.h b/include/drm/drm_mode_object.h
> index c34a3e8030e1..1906af9cae9b 100644
> --- a/include/drm/drm_mode_object.h
> +++ b/include/drm/drm_mode_object.h
> @@ -114,6 +114,9 @@ struct drm_object_properties {
> return "(unknown)"; \
> }
>
> +struct drm_mode_object *__drm_mode_object_find(struct drm_device *dev,
> + struct drm_file *file_priv,
> + u32 id, u32 type);
> struct drm_mode_object *drm_mode_object_find(struct drm_device *dev,
> struct drm_file *file_priv,
> uint32_t id, uint32_t type);
> diff --git a/include/drm/drm_plane.h b/include/drm/drm_plane.h
> index fed97e35626f..49e35d3724c9 100644
> --- a/include/drm/drm_plane.h
> +++ b/include/drm/drm_plane.h
> @@ -842,6 +842,26 @@ int drm_mode_plane_set_obj_prop(struct drm_plane *plane,
> struct drm_property *property,
> uint64_t value);
>
> +/**
> + * __drm_plane_find - find a &drm_plane
> + * @dev: DRM device
> + * @file_priv: drm file to check for lease against.
> + * @id: plane id
> + *
> + * Returns the plane with @id, NULL if it doesn't exist.
> + *
> + * Similar to drm_plane_find(), but called with &drm_device.master_rwsem held.
> + */
> +static inline struct drm_plane *__drm_plane_find(struct drm_device *dev,
> + struct drm_file *file_priv,
> + uint32_t id)
> +{
> + struct drm_mode_object *mo;
> +
> + mo = __drm_mode_object_find(dev, file_priv, id, DRM_MODE_OBJECT_PLANE);
> + return mo ? obj_to_plane(mo) : NULL;
> +}
> +
> /**
> * drm_plane_find - find a &drm_plane
> * @dev: DRM device

Aside from the one area for mode objects that cannoted be leased I think
this looks correct.

Sinc the spinlock+rwsem unification is a bit tricky, maybe you want to
split out the patch series so that we can land the initial patches
already?
-Daniel

> --
> 2.25.1
>

--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

Next message: Hans de Goede: "Re: [RFT, PATCH v2 0/2] platform/x86: hp_accel: Clean up and convert"
Previous message: Minas Harutyunyan: "Re: [PATCH] usb: dwc2: Fix spelling mistake "was't" -> "wasn't""
In reply to: Desmond Cheong Zhi Xi: "[PATCH v8 5/7] drm: avoid circular locks in drm_mode_object_find"
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