[PATCH] drivercore: fix a corner case for deferred probe

[Wei Yang]

There is one corner case in deferred probe which will lead a device in
"dream" in the deferred_probe_pending_list.

Suppose we have three devices, Tom, Jerry and Spike. Tom and Jerry have a
close relationship, Tom could be up until Jerry is up. Spike is an
independent device.

Device probe sequence: Tom -> Spike -> Jerry

1. Tom probes, fails for deferred probe
   adds itself to pending list
2. Spike probes, succeed
   move devices in pending list to active list
   trigger deferred probe
3. Tom is taken off from active list
   probes and still fails, scheduled out
   not added to pending list this time
   (Tom is not in pending list neither in active list)
4. Jerry probes, succeed
   move devices in pending list to active list(but Tom is not there)
   trigger deferred probe
   go through the active list
5. Tom add itself to pending list
   and wait

Tom will be trapped in the pending list until someone else help it out.

This patch adds a counter of success probe. Every time a driver probe succeeds,
this is increased by 1. In the deferred_probe_work_func, when probe fails and
returns EPROBE_DEFER, it checks this counter. If some driver succeed to probe
during this period, it adds itself to active list again.

Signed-off-by: Wei Yang <weiyang@xxxxxxxxxxxxxxxxxx>
---
 drivers/base/base.h |    2 +-
 drivers/base/bus.c  |    3 ++-
 drivers/base/dd.c   |   14 +++++++++++++-
 3 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/drivers/base/base.h b/drivers/base/base.h
index 24f4242..6315207 100644
--- a/drivers/base/base.h
+++ b/drivers/base/base.h
@@ -105,7 +105,7 @@ extern void container_dev_init(void);
 struct kobject *virtual_device_parent(struct device *dev);
 
 extern int bus_add_device(struct device *dev);
-extern void bus_probe_device(struct device *dev);
+extern int bus_probe_device(struct device *dev);
 extern void bus_remove_device(struct device *dev);
 
 extern int bus_add_driver(struct device_driver *drv);
diff --git a/drivers/base/bus.c b/drivers/base/bus.c
index 59dc808..a050946 100644
--- a/drivers/base/bus.c
+++ b/drivers/base/bus.c
@@ -543,7 +543,7 @@ out_put:
  *
  * - Automatically probe for a driver if the bus allows it.
  */
-void bus_probe_device(struct device *dev)
+int bus_probe_device(struct device *dev)
 {
 	struct bus_type *bus = dev->bus;
 	struct subsys_interface *sif;
@@ -562,6 +562,7 @@ void bus_probe_device(struct device *dev)
 		if (sif->add_dev)
 			sif->add_dev(dev, sif);
 	mutex_unlock(&bus->p->mutex);
+	return ret;
 }
 
 /**
diff --git a/drivers/base/dd.c b/drivers/base/dd.c
index 0605176..a10526d 100644
--- a/drivers/base/dd.c
+++ b/drivers/base/dd.c
@@ -52,6 +52,7 @@ static DEFINE_MUTEX(deferred_probe_mutex);
 static LIST_HEAD(deferred_probe_pending_list);
 static LIST_HEAD(deferred_probe_active_list);
 static struct workqueue_struct *deferred_wq;
+static u32 success_probe;
 
 /**
  * deferred_probe_work_func() - Retry probing devices in the active list.
@@ -60,6 +61,8 @@ static void deferred_probe_work_func(struct work_struct *work)
 {
 	struct device *dev;
 	struct device_private *private;
+	u32 old_success;
+	int ret = 0;
 	/*
 	 * This block processes every device in the deferred 'active' list.
 	 * Each device is removed from the active list and passed to
@@ -80,6 +83,7 @@ static void deferred_probe_work_func(struct work_struct *work)
 		list_del_init(&private->deferred_probe);
 
 		get_device(dev);
+		old_success = ACCESS_ONCE(success_probe);
 
 		/*
 		 * Drop the mutex while probing each device; the probe path may
@@ -98,7 +102,14 @@ static void deferred_probe_work_func(struct work_struct *work)
 		device_pm_unlock();
 
 		dev_dbg(dev, "Retrying from deferred list\n");
-		bus_probe_device(dev);
+		ret = bus_probe_device(dev);
+		if (ret == -EPROBE_DEFER) {
+			mutex_lock(&deferred_probe_mutex);
+			if (old_success != success_probe)
+				list_move(&private->deferred_probe,
+					  &deferred_probe_active_list);
+			mutex_unlock(&deferred_probe_mutex);
+		}
 
 		mutex_lock(&deferred_probe_mutex);
 
@@ -147,6 +158,7 @@ static void driver_deferred_probe_trigger(void)
 	 * into the active list so they can be retried by the workqueue
 	 */
 	mutex_lock(&deferred_probe_mutex);
+	success_probe++;
 	list_splice_tail_init(&deferred_probe_pending_list,
 			      &deferred_probe_active_list);
 	mutex_unlock(&deferred_probe_mutex);
-- 
1.7.9.5

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