linux-kernel - Re: [PATCH v3] driver core: Allow device link operations inside sync

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Date:   Thu, 14 Nov 2019 23:45:24 +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 v3] driver core: Allow device link operations inside sync_state()

On Thu, Nov 14, 2019 at 11:23 PM 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 | 80 ++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 72 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/base/core.c b/drivers/base/core.c
> index e6d3e6d485da..9a88bcfd5d33 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 +728,46 @@ 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 (list_empty(&dev->links.defer_sync)) {
> +               get_device(dev);
> +               list_add_tail(&dev->links.defer_sync, list);
> +       } else {
> +               WARN_ON(1);
> +       }

Now you can do:

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

        get_device(dev);
        list_add_tail(&dev->links.defer_sync, list);

The rest of the patch LGTM.