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

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