[PATCH v3] driver core: Break infinite loop when deferred probe can't be satisfied

[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.

Under "successful probe" we understand the state when a driver of the certain
device is being kept bound after deferred probe trigger cycle. For instance,
in the above mentioned case probing of tusb1210 is not successful because dwc3
driver unbinds device dwc3.0.auto.ulpi. The atomic_dec() call is used to keep
track of this. The amount of bindings is always great than or equal to the
amount of unbindings as guaranteed by design of the driver binding mechanism.

Fixes: 58b116bce136 ("drivercore: deferral race condition fix")
Suggested-by: Artem Bityutskiy <artem.bityutskiy@xxxxxxxxxxxxxxx>
Cc: Grant Likely <grant.likely@xxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Mark Brown <broonie@xxxxxxxxxx>
Cc: Felipe Balbi <balbi@xxxxxxxxxx>
Cc: Andrzej Hajda <a.hajda@xxxxxxxxxxx>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
Reviewed-by: Peter Ujfalusi <peter.ujfalusi@xxxxxx>
Tested-by: Ferry Toth <fntoth@xxxxxxxxx>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
---
v3: added comment about atomic_dec() to the commit message and code (Rafael)

 drivers/base/dd.c | 44 ++++++++++++++++++++++++++------------------
 1 file changed, 26 insertions(+), 18 deletions(-)

diff --git a/drivers/base/dd.c b/drivers/base/dd.c
index b25bcab2a26b..c1b445733150 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 +1149,13 @@ static void __device_release_driver(struct device *dev, struct device *parent)
 		dev_pm_set_driver_flags(dev, 0);
 
 		klist_remove(&dev->p->knode_driver);
+		/*
+		 * If a driver has been unbound from the device
+		 * we won't consider the probe of the device
+		 * successful.
+		 */
+		atomic_dec(&probe_okay);
+
 		device_pm_check_callbacks(dev);
 		if (dev->bus)
 			blocking_notifier_call_chain(&dev->bus->p->bus_notifier,
-- 
2.25.1