Re: [PATCH 1/2] driver core: Introduce device_link_wait_removal()

[Saravana Kannan <saravanak@google.com>]

On Thu, Nov 30, 2023 at 9:41 AM Herve Codina <herve.codina@bootlin.com> wrote:
>
> The commit 80dd33cf72d1 ("drivers: base: Fix device link removal")
> introduces a workqueue to release the consumer and supplier devices used
> in the devlink.
> In the job queued, devices are release and in turn, when all the
> references to these devices are dropped, the release function of the
> device itself is called.
>
> Nothing is present to provide some synchronisation with this workqueue
> in order to ensure that all ongoing releasing operations are done and
> so, some other operations can be started safely.
>
> For instance, in the following sequence:
>   1) of_platform_depopulate()
>   2) of_overlay_remove()
>
> During the step 1, devices are released and related devlinks are removed
> (jobs pushed in the workqueue).
> During the step 2, OF nodes are destroyed but, without any
> synchronisation with devlink removal jobs, of_overlay_remove() can raise
> warnings related to missing of_node_put():
>   ERROR: memory leak, expected refcount 1 instead of 2
>
> Indeed, the missing of_node_put() call is going to be done, too late,
> from the workqueue job execution.
>
> Introduce device_link_wait_removal() to offer a way to synchronize
> operations waiting for the end of devlink removals (i.e. end of
> workqueue jobs).
> Also, as a flushing operation is done on the workqueue, the workqueue
> used is moved from a system-wide workqueue to a local one.

Thanks for the bug report and fix. Sorry again about the delay in
reviewing the changes.

Please add Fixes tag for 80dd33cf72d1.

> Signed-off-by: Herve Codina <herve.codina@bootlin.com>
> ---
>  drivers/base/core.c    | 26 +++++++++++++++++++++++---
>  include/linux/device.h |  1 +
>  2 files changed, 24 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/base/core.c b/drivers/base/core.c
> index ac026187ac6a..2e102a77758c 100644
> --- a/drivers/base/core.c
> +++ b/drivers/base/core.c
> @@ -44,6 +44,7 @@ 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 *fw_devlink_wq;
>
>  /**
>   * __fwnode_link_add - Create a link between two fwnode_handles.
> @@ -530,12 +531,26 @@ 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.
> +        * supplier devices get deleted when it runs, so put it into the
> +        * dedicated workqueue.
>          */
> -       queue_work(system_long_wq, &link->rm_work);
> +       queue_work(fw_devlink_wq, &link->rm_work);

This has nothing to do with fw_devlink. fw_devlink is just triggering
the issue in device links. You can hit this bug without fw_devlink too.
So call this device_link_wq since it's consistent with device_link_* APIs.

>  }
>
> +/**
> + * device_link_wait_removal - Wait for ongoing devlink removal jobs to terminate
> + */
> +void device_link_wait_removal(void)
> +{
> +       /*
> +        * devlink removal jobs are queued in the dedicated work queue.
> +        * To be sure that all removal jobs are terminated, ensure that any
> +        * scheduled work has run to completion.
> +        */
> +       drain_workqueue(fw_devlink_wq);

Is there a reason this needs to be drain_workqueu() instead of
flush_workqueue(). Drain is a stronger guarantee than we need in this
case. All we are trying to make sure is that all the device link
remove work queued so far have completed.

> +}
> +EXPORT_SYMBOL_GPL(device_link_wait_removal);
> +
>  static struct class devlink_class = {
>         .name = "devlink",
>         .dev_groups = devlink_groups,
> @@ -4085,9 +4100,14 @@ int __init devices_init(void)
>         sysfs_dev_char_kobj = kobject_create_and_add("char", dev_kobj);
>         if (!sysfs_dev_char_kobj)
>                 goto char_kobj_err;
> +       fw_devlink_wq = alloc_workqueue("fw_devlink_wq", 0, 0);
> +       if (!fw_devlink_wq)

Fix the name appropriately here too please.

Thanks,
Saravana


> +               goto wq_err;
>
>         return 0;
>
> + wq_err:
> +       kobject_put(sysfs_dev_char_kobj);
>   char_kobj_err:
>         kobject_put(sysfs_dev_block_kobj);
>   block_kobj_err:
> diff --git a/include/linux/device.h b/include/linux/device.h
> index 2b093e62907a..c26f4b3df2bd 100644
> --- a/include/linux/device.h
> +++ b/include/linux/device.h
> @@ -1250,6 +1250,7 @@ void device_link_del(struct device_link *link);
>  void device_link_remove(void *consumer, struct device *supplier);
>  void device_links_supplier_sync_state_pause(void);
>  void device_links_supplier_sync_state_resume(void);
> +void device_link_wait_removal(void);
>
>  /* Create alias, so I can be autoloaded. */
>  #define MODULE_ALIAS_CHARDEV(major,minor) \
> --
> 2.42.0
>
>