Re: [driver-core PATCH v4 4/6] driver core: Probe devices asynchronously instead of the driver

[Alexander Duyck]

On 10/18/2018 11:11 AM, Bart Van Assche wrote:
> On Mon, 2018-10-15 at 08:09 -0700, Alexander Duyck wrote:
> > +static void __driver_attach_async_helper(void *_dev, async_cookie_t cookie)
> > +{
> > +	struct device *dev = _dev;
> > +
> > +	__device_driver_lock(dev, dev->parent);
> > +
> > +	/*
> > +	 * If someone attempted to bind a driver either successfully or
> > +	 * unsuccessfully before we got here we should just skip the driver
> > +	 * probe call.
> > +	 */

The answer to your question below is up here.

> > +	if (!dev->driver) {
> > +		struct device_driver *drv = dev_get_drvdata(dev);
> > +
> > +		if (drv)
> > +			driver_probe_device(drv, dev);
> > +	}
> > +
> > +	__device_driver_unlock(dev, dev->parent);
> > +
> > +	put_device(dev);
> > +
> > +	dev_dbg(dev, "async probe completed\n");
> > +}
> > +
> >   static int __driver_attach(struct device *dev, void *data)
> >   {
> >   	struct device_driver *drv = data;
> > @@ -945,6 +971,25 @@ static int __driver_attach(struct device *dev, void *data)
> >   		return ret;
> >   	} /* ret > 0 means positive match */
> >   
> > +	if (driver_allows_async_probing(drv)) {
> > +		/*
> > +		 * Instead of probing the device synchronously we will
> > +		 * probe it asynchronously to allow for more parallelism.
> > +		 *
> > +		 * We only take the device lock here in order to guarantee
> > +		 * that the dev->driver and driver_data fields are protected
> > +		 */
> > +		dev_dbg(dev, "scheduling asynchronous probe\n");
> > +		device_lock(dev);
> > +		if (!dev->driver) {
> > +			get_device(dev);
> > +			dev_set_drvdata(dev, drv);
> > +			async_schedule(__driver_attach_async_helper, dev);
> > +		}
> > +		device_unlock(dev);
> > +		return 0;
> > +	}
> > +
> >   	device_driver_attach(drv, dev);
>
> What prevents that the driver pointer becomes invalid after async_schedule() has
> been called and before __driver_attach_async_helper() is called? I think we need
> protection against concurrent driver_unregister() and __driver_attach_async_helper()
> calls. I'm not sure whether that is possible without introducing a new mutex.
>
> Thanks,
>
> Bart.

See the spot called out above.

Basically if somebody loads a driver the dev->driver becomes set. If a 
driver is removed it will clear dev->driver and set driver_data to 
0/NULL. That is what I am using as a mutex to track it in conjunction 
with the device mutex. Basically if somebody attempts to attach a driver 
before we get there we just exit and don't attempt to load this driver.