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

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Date:   Thu, 18 Oct 2018 11:11:15 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org
Cc:     len.brown@...el.com, rafael@...nel.org, linux-pm@...r.kernel.org,
        jiangshanlai@...il.com, pavel@....cz, zwisler@...nel.org,
        tj@...nel.org, akpm@...ux-foundation.org
Subject: Re: [driver-core PATCH v4 4/6] driver core: Probe devices
 asynchronously instead of the driver

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.
> +	 */
> +	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.