Date:   Thu, 14 Nov 2019 10:13:10 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Saravana Kannan <saravanak@...gle.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        "Cc: Android Kernel" <kernel-team@...roid.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] driver core: Allow device link operations inside sync_state()

On Wed, Nov 13, 2019 at 3:36 AM Saravana Kannan <saravanak@...gle.com> 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@...gle.com>
> ---
>  drivers/base/core.c | 63 +++++++++++++++++++++++++++++++++++++++------
>  1 file changed, 55 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/base/core.c b/drivers/base/core.c
> index e6d3e6d485da..d396b0597c10 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);
>  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
> + * @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)
>  {
>         struct device_link *link;
>
> @@ -709,12 +727,35 @@ 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);
> -
>         dev->state_synced = true;
> +
> +       mutex_lock(&sync_lock);

Total nit: I add empty lines around lock/unlock as a rule to make them
more visible.

> +       WARN_ON(!list_empty(&dev->links.defer_sync));
> +       if (list_empty(&dev->links.defer_sync)) {

Do you really need to duplicate that check?

> +               get_device(dev);
> +               list_add_tail(&dev->links.defer_sync, &sync_list);
> +       }
> +       mutex_unlock(&sync_lock);
> +}

What about adding

} else {
        WARN_ON(1);
}

here instead?

> +

Kerneldoc?

> +static void device_links_flush_sync_list(void)
> +{
> +       struct device *dev, *tmp;
> +
> +       mutex_lock(&sync_lock);
> +
> +       list_for_each_entry_safe(dev, tmp, &sync_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);
> +       }
> +
> +       mutex_unlock(&sync_lock);
>  }
>
>  void device_links_supplier_sync_state_pause(void)
> @@ -738,11 +779,16 @@ 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);
>         }
>  out:
>         device_links_write_unlock();
> +       device_links_flush_sync_list();

Wouldn't it be better to use a local list in this function instead of
the global sync_list?

I guess the idea is that you wouldn't be able to do the flush in
device_links_driver_bound() below, but do you really need that flush?

It looks like this is the only place calling
__device_links_queue_sync_state() and you do a flush right away after
the loop, so why is the extra flush in device_links_driver_bound()
needed?

>  }
>
>  static int sync_state_resume_initcall(void)
> @@ -815,12 +861,13 @@ 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);
>         }
>
>         dev->links.status = DL_DEV_DRIVER_BOUND;
>
>         device_links_write_unlock();
> +       device_links_flush_sync_list();

It looks like devices can be added to sync_list in parallel with each
other and so is it always OK to always flush all of them after one of
them has been bound to a driver?

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

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