Date:   Thu, 19 Mar 2020 00:47:18 +0100
From:   Ferry Toth <fntoth@...il.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        linux-kernel@...r.kernel.org
Cc:     Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>,
        Grant Likely <grant.likely@...aro.org>,
        Peter Ujfalusi <peter.ujfalusi@...com>,
        Mark Brown <broonie@...nel.org>,
        Felipe Balbi <balbi@...nel.org>,
        Andrzej Hajda <a.hajda@...sung.com>
Subject: Re: [PATCH v1 1/3] driver core: Break infinite loop when deferred
 probe can't be satisfied

Op 09-03-2020 om 15:11 schreef Andy Shevchenko:
> Consider the following scenario.
> 
> The main driver of USB OTG controller (dwc3-pci), which has the following
> functional dependencies on certain platform:
> - ULPI (tusb1210)
> - extcon (tested with extcon-intel-mrfld)
> 
> Note, that first driver, tusb1210, is available at the moment of
> dwc3-pci probing, while extcon-intel-mrfld is built as a module and
> won't appear till user space does something about it.
> 
> This is depicted by kernel configuration excerpt:
> 
> 	CONFIG_PHY_TUSB1210=y
> 	CONFIG_USB_DWC3=y
> 	CONFIG_USB_DWC3_ULPI=y
> 	CONFIG_USB_DWC3_DUAL_ROLE=y
> 	CONFIG_USB_DWC3_PCI=y
> 	CONFIG_EXTCON_INTEL_MRFLD=m
> 
> In the Buildroot environment the modules are probed by alphabetical ordering
> of their modaliases. The latter comes to the case when USB OTG driver will be
> probed first followed by extcon one.
> 
> So, if the platform anticipates extcon device to be appeared, in the above case
> we will get deferred probe of USB OTG, because of ordering.
> 
> Since current implementation, done by the commit 58b116bce136 ("drivercore:
> deferral race condition fix") counts the amount of triggered deferred probe,
> we never advance the situation -- the change makes it to be an infinite loop.
> 
> ---8<---8<---
> 
> [   22.187127] driver_deferred_probe_trigger <<< 1
> 
> ...here is the late initcall triggers deferred probe...
> 
> [   22.191725] platform dwc3.0.auto: deferred_probe_work_func in deferred list
> 
> ...dwc3.0.auto is the only device in the deferred list...
> 
> [   22.198727] platform dwc3.0.auto: deferred_probe_work_func 1 <<< counter 1
> 
> ...the counter before mutex is unlocked is kept the same...
> 
> [   22.205663] platform dwc3.0.auto: Retrying from deferred list
> 
> ...mutes has been unlocked, we try to re-probe the driver...
> 
> [   22.211487] bus: 'platform': driver_probe_device: matched device dwc3.0.auto with driver dwc3
> [   22.220060] bus: 'platform': really_probe: probing driver dwc3 with device dwc3.0.auto
> [   22.238735] bus: 'ulpi': driver_probe_device: matched device dwc3.0.auto.ulpi with driver tusb1210
> [   22.247743] bus: 'ulpi': really_probe: probing driver tusb1210 with device dwc3.0.auto.ulpi
> [   22.256292] driver: 'tusb1210': driver_bound: bound to device 'dwc3.0.auto.ulpi'
> [   22.263723] driver_deferred_probe_trigger <<< 2
> 
> ...the dwc3.0.auto probes ULPI, we got successful bound and bumped counter...
> 
> [   22.268304] bus: 'ulpi': really_probe: bound device dwc3.0.auto.ulpi to driver tusb1210
> [   22.276697] platform dwc3.0.auto: Driver dwc3 requests probe deferral
> 
> ...but extcon driver is still missing...
> 
> [   22.283174] platform dwc3.0.auto: Added to deferred list
> [   22.288513] platform dwc3.0.auto: driver_deferred_probe_add_trigger local counter: 1 new counter 2
> 
> ...and since we had a successful probe, we got counter mismatch...
> 
> [   22.297490] driver_deferred_probe_trigger <<< 3
> [   22.302074] platform dwc3.0.auto: deferred_probe_work_func 2 <<< counter 3
> 
> ...at the end we have a new counter and loop repeats again, see 22.198727...
> 
> ---8<---8<---
> 
> Revert of the commit helps, but it is probably not helpful for the initially
> found regression. Artem Bityutskiy suggested to use counter of the successful
> probes instead. This fixes above mentioned case and shouldn't prevent driver
> to reprobe deferred ones.
> 
> Fixes: 58b116bce136 ("drivercore: deferral race condition fix")
> Suggested-by: Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>
> Cc: Grant Likely <grant.likely@...aro.org>
> Cc: Peter Ujfalusi <peter.ujfalusi@...com>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Mark Brown <broonie@...nel.org>
> Cc: Felipe Balbi <balbi@...nel.org>
> Cc: Andrzej Hajda <a.hajda@...sung.com>
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>

Tested-by: Ferry Toth <fntoth@...il.com>

> ---
>   drivers/base/dd.c | 39 +++++++++++++++++++++------------------
>   1 file changed, 21 insertions(+), 18 deletions(-)
> 
> diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> index b25bcab2a26b..43720beb5300 100644
> --- a/drivers/base/dd.c
> +++ b/drivers/base/dd.c
> @@ -53,7 +53,6 @@
>   static DEFINE_MUTEX(deferred_probe_mutex);
>   static LIST_HEAD(deferred_probe_pending_list);
>   static LIST_HEAD(deferred_probe_active_list);
> -static atomic_t deferred_trigger_count = ATOMIC_INIT(0);
>   static struct dentry *deferred_devices;
>   static bool initcalls_done;
>   
> @@ -147,17 +146,6 @@ static bool driver_deferred_probe_enable = false;
>    * This functions moves all devices from the pending list to the active
>    * list and schedules the deferred probe workqueue to process them.  It
>    * should be called anytime a driver is successfully bound to a device.
> - *
> - * Note, there is a race condition in multi-threaded probe. In the case where
> - * more than one device is probing at the same time, it is possible for one
> - * probe to complete successfully while another is about to defer. If the second
> - * depends on the first, then it will get put on the pending list after the
> - * trigger event has already occurred and will be stuck there.
> - *
> - * The atomic 'deferred_trigger_count' is used to determine if a successful
> - * trigger has occurred in the midst of probing a driver. If the trigger count
> - * changes in the midst of a probe, then deferred processing should be triggered
> - * again.
>    */
>   static void driver_deferred_probe_trigger(void)
>   {
> @@ -170,7 +158,6 @@ static void driver_deferred_probe_trigger(void)
>   	 * into the active list so they can be retried by the workqueue
>   	 */
>   	mutex_lock(&deferred_probe_mutex);
> -	atomic_inc(&deferred_trigger_count);
>   	list_splice_tail_init(&deferred_probe_pending_list,
>   			      &deferred_probe_active_list);
>   	mutex_unlock(&deferred_probe_mutex);
> @@ -350,6 +337,19 @@ static void __exit deferred_probe_exit(void)
>   }
>   __exitcall(deferred_probe_exit);
>   
> +/*
> + * Note, there is a race condition in multi-threaded probe. In the case where
> + * more than one device is probing at the same time, it is possible for one
> + * probe to complete successfully while another is about to defer. If the second
> + * depends on the first, then it will get put on the pending list after the
> + * trigger event has already occurred and will be stuck there.
> + *
> + * The atomic 'probe_okay' is used to determine if a successful probe has
> + * occurred in the midst of probing another driver. If the count changes in
> + * the midst of a probe, then deferred processing should be triggered again.
> + */
> +static atomic_t probe_okay = ATOMIC_INIT(0);
> +
>   /**
>    * device_is_bound() - Check if device is bound to a driver
>    * @dev: device to check
> @@ -375,6 +375,7 @@ static void driver_bound(struct device *dev)
>   	pr_debug("driver: '%s': %s: bound to device '%s'\n", dev->driver->name,
>   		 __func__, dev_name(dev));
>   
> +	atomic_inc(&probe_okay);
>   	klist_add_tail(&dev->p->knode_driver, &dev->driver->p->klist_devices);
>   	device_links_driver_bound(dev);
>   
> @@ -481,18 +482,18 @@ static atomic_t probe_count = ATOMIC_INIT(0);
>   static DECLARE_WAIT_QUEUE_HEAD(probe_waitqueue);
>   
>   static void driver_deferred_probe_add_trigger(struct device *dev,
> -					      int local_trigger_count)
> +					      int local_probe_okay_count)
>   {
>   	driver_deferred_probe_add(dev);
>   	/* Did a trigger occur while probing? Need to re-trigger if yes */
> -	if (local_trigger_count != atomic_read(&deferred_trigger_count))
> +	if (local_probe_okay_count != atomic_read(&probe_okay))
>   		driver_deferred_probe_trigger();
>   }
>   
>   static int really_probe(struct device *dev, struct device_driver *drv)
>   {
>   	int ret = -EPROBE_DEFER;
> -	int local_trigger_count = atomic_read(&deferred_trigger_count);
> +	int local_probe_okay_count = atomic_read(&probe_okay);
>   	bool test_remove = IS_ENABLED(CONFIG_DEBUG_TEST_DRIVER_REMOVE) &&
>   			   !drv->suppress_bind_attrs;
>   
> @@ -509,7 +510,7 @@ static int really_probe(struct device *dev, struct device_driver *drv)
>   
>   	ret = device_links_check_suppliers(dev);
>   	if (ret == -EPROBE_DEFER)
> -		driver_deferred_probe_add_trigger(dev, local_trigger_count);
> +		driver_deferred_probe_add_trigger(dev, local_probe_okay_count);
>   	if (ret)
>   		return ret;
>   
> @@ -619,7 +620,7 @@ static int really_probe(struct device *dev, struct device_driver *drv)
>   	case -EPROBE_DEFER:
>   		/* Driver requested deferred probing */
>   		dev_dbg(dev, "Driver %s requests probe deferral\n", drv->name);
> -		driver_deferred_probe_add_trigger(dev, local_trigger_count);
> +		driver_deferred_probe_add_trigger(dev, local_probe_okay_count);
>   		break;
>   	case -ENODEV:
>   	case -ENXIO:
> @@ -1148,6 +1149,8 @@ static void __device_release_driver(struct device *dev, struct device *parent)
>   		dev_pm_set_driver_flags(dev, 0);
>   
>   		klist_remove(&dev->p->knode_driver);
> +		atomic_dec(&probe_okay);
> +
>   		device_pm_check_callbacks(dev);
>   		if (dev->bus)
>   			blocking_notifier_call_chain(&dev->bus->p->bus_notifier,
> 

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