Re: [RFC v1 0/3] driver-core: add asynch module loading support

[David Herrmann]

Hi

On Sun, Aug 31, 2014 at 1:02 PM, Tejun Heo <tj@xxxxxxxxxx> wrote:
> So, something like the following.  A couple things to note
>
> * driver_attach() can never fail but is marked with __must_check.  We
>   prolly should change it to void.
>
> * Old/weird userspace which depends on insmod to wait for device
>   probing might choke and the new behavior might need to be switched
>   somehow (sysctl, insmod param or whatever).
>
> Lightly tested and seems to work fine.
>
> Thanks.
> ---
>  drivers/base/bus.c |   39 ++++++++++++++++++++++++++++++++++++---
>  1 file changed, 36 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/base/bus.c b/drivers/base/bus.c
> index 83e910a..e4fc9bc 100644
> --- a/drivers/base/bus.c
> +++ b/drivers/base/bus.c
> @@ -23,6 +23,7 @@
>
>  /* /sys/devices/system */
>  static struct kset *system_kset;
> +static struct workqueue_struct *driver_attach_wq;
>
>  #define to_bus_attr(_attr) container_of(_attr, struct bus_attribute, attr)
>
> @@ -657,6 +658,20 @@ static ssize_t uevent_store(struct device_driver *drv, const char *buf,
>  }
>  static DRIVER_ATTR_WO(uevent);
>
> +struct driver_attach_work {
> +       struct work_struct              work;
> +       struct device_driver            *driver;
> +};
> +
> +static void driver_attach_workfn(struct work_struct *work)
> +{
> +       struct driver_attach_work *daw =
> +               container_of(work, struct driver_attach_work, work);
> +
> +       driver_attach(daw->driver);
> +       kfree(daw);
> +}
> +
>  /**
>   * bus_add_driver - Add a driver to the bus.
>   * @drv: driver.
> @@ -689,9 +704,23 @@ int bus_add_driver(struct device_driver *drv)
>
>         klist_add_tail(&priv->knode_bus, &bus->p->klist_drivers);
>         if (drv->bus->p->drivers_autoprobe) {
> -               error = driver_attach(drv);
> -               if (error)
> -                       goto out_unregister;
> +               struct driver_attach_work *daw;
> +
> +               if (drv->owner) {
> +                       daw = kzalloc(sizeof(*daw), GFP_KERNEL);
> +                       if (!daw) {
> +                               error = -ENOMEM;
> +                               goto out_unregister;
> +                       }
> +
> +                       INIT_WORK(&daw->work, driver_attach_workfn);
> +                       daw->driver = drv;
> +                       queue_work(driver_attach_wq, &daw->work);

Doesn't this break on-demand cdev initialization? We currently call
request_module() on open() for unclaimed major/minor combinations. If
driver_attach() is no longer part of module_init(), there is no
guarantee the driver created the cdev before request_module() returns.

I actually like this "deferred attach" approach, so this is not meant
as counter-argument. We just need to make sure to have a notion of
"settled modules" so we know how long to wait after loading a module.

Thanks
David

> +               } else {
> +                       error = driver_attach(drv);
> +                       if (error)
> +                               goto out_unregister;
> +               }
>         }
>         module_add_driver(drv->owner, drv);
>
> @@ -1268,5 +1297,9 @@ int __init buses_init(void)
>         if (!system_kset)
>                 return -ENOMEM;
>
> +       driver_attach_wq = alloc_ordered_workqueue("driver_attach", 0);
> +       if (!driver_attach_wq)
> +               return -ENOMEM;
> +
>         return 0;
>  }
> --
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