Re: [PATCH 1/2] driver: core: add dedicated workqueue for devlink removal

["Rafael J. Wysocki" <rafael@kernel.org>]

On Fri, Feb 2, 2024 at 1:18 PM Nuno Sa via B4 Relay
<devnull+nuno.sa.analog.com@kernel.org> wrote:
>
> From: Nuno Sa <nuno.sa@analog.com>
>
> Let's use a dedicated queue for devlinks since releasing a link happens
> asynchronously but some code paths, like DT overlays, have some
> expectations regarding the of_node when being removed (the refcount must
> be 1). Given how devlinks are released that cannot be assured. Hence, add a
> dedicated queue so that it's easy to sync against devlinks removal.

Thanks for following my suggestion!

> While at it, make sure to explicitly include <linux/workqueue.h>.
>
> Signed-off-by: Nuno Sa <nuno.sa@analog.com>
> ---
>  drivers/base/core.c    | 33 +++++++++++++++++++++++++++++----
>  include/linux/fwnode.h |  1 +
>  2 files changed, 30 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/base/core.c b/drivers/base/core.c
> index 14d46af40f9a..06e7766b5227 100644
> --- a/drivers/base/core.c
> +++ b/drivers/base/core.c
> @@ -31,6 +31,7 @@
>  #include <linux/swiotlb.h>
>  #include <linux/sysfs.h>
>  #include <linux/dma-map-ops.h> /* for dma_default_coherent */
> +#include <linux/workqueue.h>
>
>  #include "base.h"
>  #include "physical_location.h"
> @@ -44,6 +45,7 @@ static bool fw_devlink_is_permissive(void);
>  static void __fw_devlink_link_to_consumers(struct device *dev);
>  static bool fw_devlink_drv_reg_done;
>  static bool fw_devlink_best_effort;
> +static struct workqueue_struct *devlink_release_queue __ro_after_init;
>
>  /**
>   * __fwnode_link_add - Create a link between two fwnode_handles.
> @@ -235,6 +237,11 @@ static void __fw_devlink_pickup_dangling_consumers(struct fwnode_handle *fwnode,
>                 __fw_devlink_pickup_dangling_consumers(child, new_sup);
>  }
>
> +void fwnode_links_flush_queue(void)
> +{
> +       flush_workqueue(devlink_release_queue);
> +}
> +
>  static DEFINE_MUTEX(device_links_lock);
>  DEFINE_STATIC_SRCU(device_links_srcu);
>
> @@ -531,9 +538,10 @@ static void devlink_dev_release(struct device *dev)
>          * It may take a while to complete this work because of the SRCU
>          * synchronization in device_link_release_fn() and if the consumer or
>          * supplier devices get deleted when it runs, so put it into the "long"
> -        * workqueue.
> +        * devlink workqueue.
> +        *
>          */
> -       queue_work(system_long_wq, &link->rm_work);
> +       queue_work(devlink_release_queue, &link->rm_work);
>  }
>
>  static struct class devlink_class = {
> @@ -636,10 +644,27 @@ static int __init devlink_class_init(void)
>                 return ret;
>
>         ret = class_interface_register(&devlink_class_intf);
> -       if (ret)
> +       if (ret) {
> +               class_unregister(&devlink_class);
> +               return ret;
> +       }
> +
> +       /*
> +        * Using a dedicated queue for devlinks since releasing a link happens
> +        * asynchronously but some code paths, like DT overlays, have some
> +        * expectations regarding the of_node when being removed (the refcount
> +        * must be 1). Given how devlinks are released that cannot be assured.
> +        * Hence, add a dedicated queue so that it's easy to sync against
> +        * devlinks removal.
> +        */
> +       devlink_release_queue = alloc_workqueue("devlink_release", 0, 0);
> +       if (!devlink_release_queue) {
> +               class_interface_unregister(&devlink_class_intf);
>                 class_unregister(&devlink_class);

This is a bit drastic.

I think that device links can still work if devlink_release_queue is
NULL, just devlink_dev_release() needs to check it and release
synchronously if it is NULL.

Apart from this LGTM.

> +               return -ENODEV;
> +       }
>
> -       return ret;
> +       return 0;
>  }
>  postcore_initcall(devlink_class_init);
>
> diff --git a/include/linux/fwnode.h b/include/linux/fwnode.h
> index 2a72f55d26eb..017b170e9903 100644
> --- a/include/linux/fwnode.h
> +++ b/include/linux/fwnode.h
> @@ -213,5 +213,6 @@ extern bool fw_devlink_is_strict(void);
>  int fwnode_link_add(struct fwnode_handle *con, struct fwnode_handle *sup);
>  void fwnode_links_purge(struct fwnode_handle *fwnode);
>  void fw_devlink_purge_absent_suppliers(struct fwnode_handle *fwnode);
> +void fwnode_links_flush_queue(void);
>
>  #endif
>
> --