Re: [driver-core PATCH v5 5/9] driver core: Establish clear order of operations for deferred probe and remove

[Dan Williams]

On Mon, Nov 5, 2018 at 1:12 PM Alexander Duyck
<alexander.h.duyck@xxxxxxxxxxxxxxx> wrote:
>
> This patch adds an additional bit to the device struct named async_probe.
> This additional bit allows us to guarantee ordering between probe and
> remove operations.
>
> This allows us to guarantee that if we execute a remove operation or a
> driver load attempt on a given interface it will not attempt to update the
> driver member asynchronously following the earlier operation. Previously
> this guarantee was not present and could result in us attempting to remove
> a driver from an interface only to have it show up later when it is
> asynchronously loaded.
>
> One change I made in addition is I replaced the use of "bool X:1" to define
> the bitfield to a "u8 X:1" setup in order to resolve some checkpatch
> warnings.

The usage of "us" in the changelog through me off, please reword this
to explicitly state the subject like: "The additional bit allows the
driver core to guarantee ordering between probe and remove
operations."

> Signed-off-by: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
> ---
>  drivers/base/dd.c      |  104 +++++++++++++++++++++++++++---------------------
>  include/linux/device.h |    9 ++--
>  2 files changed, 64 insertions(+), 49 deletions(-)
>
> diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> index e74cefeb5b69..ed19cf0d6f9a 100644
> --- a/drivers/base/dd.c
> +++ b/drivers/base/dd.c
> @@ -472,6 +472,8 @@ static int really_probe(struct device *dev, struct device_driver *drv)
>                  drv->bus->name, __func__, drv->name, dev_name(dev));
>         WARN_ON(!list_empty(&dev->devres_head));
>
> +       /* clear async_probe flag as we are no longer deferring driver load */
> +       dev->async_probe = false;
>  re_probe:
>         dev->driver = drv;
>
> @@ -771,6 +773,10 @@ static void __device_attach_async_helper(void *_dev, async_cookie_t cookie)
>
>         device_lock(dev);
>
> +       /* nothing to do if async_probe has been cleared */
> +       if (!dev->async_probe)
> +               goto out_unlock;
> +
>         if (dev->parent)
>                 pm_runtime_get_sync(dev->parent);
>
> @@ -781,7 +787,7 @@ static void __device_attach_async_helper(void *_dev, async_cookie_t cookie)
>
>         if (dev->parent)
>                 pm_runtime_put(dev->parent);
> -
> +out_unlock:
>         device_unlock(dev);
>
>         put_device(dev);
> @@ -826,6 +832,7 @@ static int __device_attach(struct device *dev, bool allow_async)
>                          */
>                         dev_dbg(dev, "scheduling asynchronous probe\n");
>                         get_device(dev);
> +                       dev->async_probe = true;
>                         async_schedule(__device_attach_async_helper, dev);
>                 } else {
>                         pm_request_idle(dev);
> @@ -971,62 +978,69 @@ EXPORT_SYMBOL_GPL(driver_attach);
>   */
>  static void __device_release_driver(struct device *dev, struct device *parent)
>  {
> -       struct device_driver *drv;
> +       struct device_driver *drv = dev->driver;
>
> -       drv = dev->driver;
> -       if (drv) {
> -               while (device_links_busy(dev)) {
> -                       __device_driver_unlock(dev, parent);
> +       /*
> +        * In the event that we are asked to release the driver on an
> +        * interface that is still waiting on a probe we can just terminate
> +        * the probe by setting async_probe to false. When the async call
> +        * is finally completed it will see this state and just exit.
> +        */
> +       dev->async_probe = false;
> +       if (!drv)
> +               return;

Patch 4 deleted the async_synchronize_full() that would have flushed
in-flight ->probe() relative to the current ->remove(). If remove runs
before probe then would seem to be deadlock condition, but if
->remove() runs before probe then dev->driver is NULL and we abort. So
I'm struggling to see what value dev->async_probe provides over
dev->driver?