Re: [PATCH v2] driver core: Allow device link operations inside sync_state()

[Rafael J. Wysocki]

On Thu, Nov 14, 2019 at 8:42 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>

It would have been kind of nice to let me respond to your last reply
before sending this.  Oh well.

> ---
>  drivers/base/core.c | 76 ++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 68 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/base/core.c b/drivers/base/core.c
> index e6d3e6d485da..2f14d4bf1472 100644
> --- a/drivers/base/core.c
> +++ b/drivers/base/core.c
> @@ -48,6 +48,8 @@ early_param("sysfs.deprecated", sysfs_deprecated_setup);
>  static LIST_HEAD(wait_for_suppliers);
>  static DEFINE_MUTEX(wfs_lock);
>  static LIST_HEAD(deferred_sync);
> +static LIST_HEAD(sync_list);
> +static DEFINE_MUTEX(sync_lock);

The two items above are not needed AFAICS.

>  static unsigned int defer_sync_state_count = 1;
>
>  #ifdef CONFIG_SRCU
> @@ -695,7 +697,23 @@ 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

This should be

/**
 * __device_links_queue_sync_state - Queue a device for sync_state() callback

> + * @dev: Device to call sync_state() 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)

Pass a list head as a second arg.

>  {
>         struct device_link *link;
>
> @@ -709,12 +727,48 @@ 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 the sync_list
> +        * more than once. This can happen if new consumers get added to the
> +        * device before the sync_list is flushed.
> +        */
>         dev->state_synced = true;
> +
> +       mutex_lock(&sync_lock);
> +
> +       if (list_empty(&dev->links.defer_sync)) {
> +               get_device(dev);
> +               list_add_tail(&dev->links.defer_sync, &sync_list);

Add it to the list that you have passed as the second arg.  No locking.

> +       } else {
> +               WARN_ON(1);
> +       }
> +
> +       mutex_unlock(&sync_lock);
> +}
> +
> +/** device_links_flush_sync_list - Call sync_state() on devices queued for it
> + *
> + * 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(void)

Make it take a list to flush as an arg.

> +{
> +       struct device *dev, *tmp;
> +
> +       mutex_lock(&sync_lock);

This isn't necessary.

> +
> +       list_for_each_entry_safe(dev, tmp, &sync_list, links.defer_sync) {
> +               list_del_init(&dev->links.defer_sync);
> +               device_lock(dev);

Empty lines around this?

> +               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);

And this?

> +               put_device(dev);
> +       }
> +
> +       mutex_unlock(&sync_lock);

This isn't necessary.

>  }
>
>  void device_links_supplier_sync_state_pause(void)
> @@ -738,11 +792,16 @@ 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);

Use a local list (initially empty) in this function and pass it to the above.

>         }
>  out:
>         device_links_write_unlock();
> +       device_links_flush_sync_list();

Pass the local list here too.

>  }
>
>  static int sync_state_resume_initcall(void)
> @@ -815,12 +874,13 @@ 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);

Like in the previous case, use a local list (initially empty) in this
function and pass it to the above.

>         }
>
>         dev->links.status = DL_DEV_DRIVER_BOUND;
>
>         device_links_write_unlock();
> +       device_links_flush_sync_list();

And pass that list here too.

>  }
>
>  static void device_link_drop_managed(struct device_link *link)
> --