Re: [PATCH v4] driver core: Allow device link operations inside sync_state()

[Rafael J. Wysocki]

On Thu, Nov 14, 2019 at 11:56 PM Saravana Kannan <saravanak@xxxxxxxxxx> wrote:
>
> Some sync_state() implementations might need to call APIs that in turn
> make calls to device link APIs. So, do the sync_state() callbacks
> without holding the device link lock.
>
> Signed-off-by: Saravana Kannan <saravanak@xxxxxxxxxx>

All of my comments have been addressed, so:

Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>

> ---
>  drivers/base/core.c | 79 ++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 71 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/base/core.c b/drivers/base/core.c
> index e6d3e6d485da..42a672456432 100644
> --- a/drivers/base/core.c
> +++ b/drivers/base/core.c
> @@ -695,7 +695,26 @@ int device_links_check_suppliers(struct device *dev)
>         return ret;
>  }
>
> -static void __device_links_supplier_sync_state(struct device *dev)
> +/**
> + * __device_links_queue_sync_state - Queue a device for sync_state() callback
> + * @dev: Device to call sync_state() on
> + * @list: List head to queue the @dev on
> + *
> + * Queues a device for a sync_state() callback when the device links write lock
> + * isn't held. This allows the sync_state() execution flow to use device links
> + * APIs.  The caller must ensure this function is called with
> + * device_links_write_lock() held.
> + *
> + * This function does a get_device() to make sure the device is not freed while
> + * on this list.
> + *
> + * So the caller must also ensure that device_links_flush_sync_list() is called
> + * as soon as the caller releases device_links_write_lock().  This is necessary
> + * to make sure the sync_state() is called in a timely fashion and the
> + * put_device() is called on this device.
> + */
> +static void __device_links_queue_sync_state(struct device *dev,
> +                                           struct list_head *list)
>  {
>         struct device_link *link;
>
> @@ -709,12 +728,45 @@ static void __device_links_supplier_sync_state(struct device *dev)
>                         return;
>         }
>
> -       if (dev->bus->sync_state)
> -               dev->bus->sync_state(dev);
> -       else if (dev->driver && dev->driver->sync_state)
> -               dev->driver->sync_state(dev);
> -
> +       /*
> +        * Set the flag here to avoid adding the same device to a list more
> +        * than once. This can happen if new consumers get added to the device
> +        * and probed before the list is flushed.
> +        */
>         dev->state_synced = true;
> +
> +       if (WARN_ON(!list_empty(&dev->links.defer_sync)))
> +               return;
> +
> +       get_device(dev);
> +       list_add_tail(&dev->links.defer_sync, list);
> +}
> +
> +/**
> + * device_links_flush_sync_list - Call sync_state() on a list of devices
> + * @list: List of devices to call sync_state() on
> + *
> + * Calls sync_state() on all the devices that have been queued for it. This
> + * function is used in conjunction with __device_links_queue_sync_state().
> + */
> +static void device_links_flush_sync_list(struct list_head *list)
> +{
> +       struct device *dev, *tmp;
> +
> +       list_for_each_entry_safe(dev, tmp, list, links.defer_sync) {
> +               list_del_init(&dev->links.defer_sync);
> +
> +               device_lock(dev);
> +
> +               if (dev->bus->sync_state)
> +                       dev->bus->sync_state(dev);
> +               else if (dev->driver && dev->driver->sync_state)
> +                       dev->driver->sync_state(dev);
> +
> +               device_unlock(dev);
> +
> +               put_device(dev);
> +       }
>  }
>
>  void device_links_supplier_sync_state_pause(void)
> @@ -727,6 +779,7 @@ void device_links_supplier_sync_state_pause(void)
>  void device_links_supplier_sync_state_resume(void)
>  {
>         struct device *dev, *tmp;
> +       LIST_HEAD(sync_list);
>
>         device_links_write_lock();
>         if (!defer_sync_state_count) {
> @@ -738,11 +791,17 @@ void device_links_supplier_sync_state_resume(void)
>                 goto out;
>
>         list_for_each_entry_safe(dev, tmp, &deferred_sync, links.defer_sync) {
> -               __device_links_supplier_sync_state(dev);
> +               /*
> +                * Delete from deferred_sync list before queuing it to
> +                * sync_list because defer_sync is used for both lists.
> +                */
>                 list_del_init(&dev->links.defer_sync);
> +               __device_links_queue_sync_state(dev, &sync_list);
>         }
>  out:
>         device_links_write_unlock();
> +
> +       device_links_flush_sync_list(&sync_list);
>  }
>
>  static int sync_state_resume_initcall(void)
> @@ -772,6 +831,7 @@ static void __device_links_supplier_defer_sync(struct device *sup)
>  void device_links_driver_bound(struct device *dev)
>  {
>         struct device_link *link;
> +       LIST_HEAD(sync_list);
>
>         /*
>          * If a device probes successfully, it's expected to have created all
> @@ -815,12 +875,15 @@ void device_links_driver_bound(struct device *dev)
>                 if (defer_sync_state_count)
>                         __device_links_supplier_defer_sync(link->supplier);
>                 else
> -                       __device_links_supplier_sync_state(link->supplier);
> +                       __device_links_queue_sync_state(link->supplier,
> +                                                       &sync_list);
>         }
>
>         dev->links.status = DL_DEV_DRIVER_BOUND;
>
>         device_links_write_unlock();
> +
> +       device_links_flush_sync_list(&sync_list);
>  }
>
>  static void device_link_drop_managed(struct device_link *link)
> --
> 2.24.0.432.g9d3f5f5b63-goog
>