counter: Provide alternative counter registration functions

The current implementation gets device lifetime tracking wrong. The
problem is that allocation of struct counter_device is controlled by the
individual drivers but this structure contains a struct device that
might have to live longer than a driver is bound. As a result a command
sequence like:

	{ sleep 5; echo bang; } > /dev/counter0 &
	sleep 1;
	echo 40000000.timer:counter > /sys/bus/platform/drivers/stm32-timer-counter/unbind

can keep a reference to the struct device and unbinding results in
freeing the memory occupied by this device resulting in an oops.

This commit provides two new functions (plus some helpers):
 - counter_alloc() to allocate a struct counter_device that is
   automatically freed once the embedded struct device is released
 - counter_add() to register such a device.

Note that this commit doesn't fix any issues, all drivers have to be
converted to these new functions to correct the lifetime problems.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20211230150300.72196-14-u.kleine-koenig@pengutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/counter/counter-core.c

@@ -15,6 +15,7 @@
 #include <linux/kdev_t.h>
 #include <linux/module.h>
 #include <linux/mutex.h>
+#include <linux/slab.h>
 #include <linux/types.h>
 #include <linux/wait.h>
 
@@ -24,13 +25,26 @@
 /* Provides a unique ID for each counter device */
 static DEFINE_IDA(counter_ida);
 
+struct counter_device_allochelper {
+	struct counter_device counter;
+
+	/*
+	 * This is cache line aligned to ensure private data behaves like if it
+	 * were kmalloced separately.
+	 */
+	unsigned long privdata[] ____cacheline_aligned;
+};
+
 static void counter_device_release(struct device *dev)
 {
 	struct counter_device *const counter =
 		container_of(dev, struct counter_device, dev);
 
 	counter_chrdev_remove(counter);
 	ida_free(&counter_ida, dev->id);
+
+	if (!counter->legacy_device)
+		kfree(container_of(counter, struct counter_device_allochelper, counter));
 }
 
 static struct device_type counter_device_type = {
@@ -53,7 +67,14 @@ static dev_t counter_devt;
  */
 void *counter_priv(const struct counter_device *const counter)
 {
-	return counter->priv;
+	if (counter->legacy_device) {
+		return counter->priv;
+	} else {
+		struct counter_device_allochelper *ch =
+			container_of(counter, struct counter_device_allochelper, counter);
+
+		return &ch->privdata;
+	}
 }
 EXPORT_SYMBOL_GPL(counter_priv);
 
@@ -74,6 +95,8 @@ int counter_register(struct counter_device *const counter)
 	int id;
 	int err;
 
+	counter->legacy_device = true;
+
 	/* Acquire unique ID */
 	id = ida_alloc(&counter_ida, GFP_KERNEL);
 	if (id < 0)
@@ -114,6 +137,95 @@ int counter_register(struct counter_device *const counter)
 }
 EXPORT_SYMBOL_GPL(counter_register);
 
+/**
+ * counter_alloc - allocate a counter_device
+ * @sizeof_priv: size of the driver private data
+ *
+ * This is part one of counter registration. The structure is allocated
+ * dynamically to ensure the right lifetime for the embedded struct device.
+ *
+ * If this succeeds, call counter_put() to get rid of the counter_device again.
+ */
+struct counter_device *counter_alloc(size_t sizeof_priv)
+{
+	struct counter_device_allochelper *ch;
+	struct counter_device *counter;
+	struct device *dev;
+	int err;
+
+	ch = kzalloc(sizeof(*ch) + sizeof_priv, GFP_KERNEL);
+	if (!ch) {
+		err = -ENOMEM;
+		goto err_alloc_ch;
+	}
+
+	counter = &ch->counter;
+	dev = &counter->dev;
+
+	/* Acquire unique ID */
+	err = ida_alloc(&counter_ida, GFP_KERNEL);
+	if (err < 0)
+		goto err_ida_alloc;
+	dev->id = err;
+
+	mutex_init(&counter->ops_exist_lock);
+	dev->type = &counter_device_type;
+	dev->bus = &counter_bus_type;
+	dev->devt = MKDEV(MAJOR(counter_devt), dev->id);
+
+	err = counter_chrdev_add(counter);
+	if (err < 0)
+		goto err_chrdev_add;
+
+	device_initialize(dev);
+
+	return counter;
+
+err_chrdev_add:
+
+	ida_free(&counter_ida, dev->id);
+err_ida_alloc:
+
+	kfree(ch);
+err_alloc_ch:
+
+	return ERR_PTR(err);
+}
+EXPORT_SYMBOL_GPL(counter_alloc);
+
+void counter_put(struct counter_device *counter)
+{
+	put_device(&counter->dev);
+}
+EXPORT_SYMBOL_GPL(counter_put);
+
+/**
+ * counter_add - complete registration of a counter
+ * @counter: the counter to add
+ *
+ * This is part two of counter registration.
+ *
+ * If this succeeds, call counter_unregister() to get rid of the counter_device again.
+ */
+int counter_add(struct counter_device *counter)
+{
+	int err;
+	struct device *dev = &counter->dev;
+
+	if (counter->parent) {
+		dev->parent = counter->parent;
+		dev->of_node = counter->parent->of_node;
+	}
+
+	err = counter_sysfs_add(counter);
+	if (err < 0)
+		return err;
+
+	/* implies device_add(dev) */
+	return cdev_device_add(&counter->chrdev, dev);
+}
+EXPORT_SYMBOL_GPL(counter_add);
+
 /**
  * counter_unregister - unregister Counter from the system
  * @counter:	pointer to Counter to unregister
@@ -134,7 +246,8 @@ void counter_unregister(struct counter_device *const counter)
 
 	mutex_unlock(&counter->ops_exist_lock);
 
-	put_device(&counter->dev);
+	if (counter->legacy_device)
+		put_device(&counter->dev);
 }
 EXPORT_SYMBOL_GPL(counter_unregister);
 
@@ -168,6 +281,57 @@ int devm_counter_register(struct device *dev,
 }
 EXPORT_SYMBOL_GPL(devm_counter_register);
 
+static void devm_counter_put(void *counter)
+{
+	counter_put(counter);
+}
+
+/**
+ * devm_counter_alloc - allocate a counter_device
+ * @dev: the device to register the release callback for
+ * @sizeof_priv: size of the driver private data
+ *
+ * This is the device managed version of counter_add(). It registers a cleanup
+ * callback to care for calling counter_put().
+ */
+struct counter_device *devm_counter_alloc(struct device *dev, size_t sizeof_priv)
+{
+	struct counter_device *counter;
+	int err;
+
+	counter = counter_alloc(sizeof_priv);
+	if (IS_ERR(counter))
+		return counter;
+
+	err = devm_add_action_or_reset(dev, devm_counter_put, counter);
+	if (err < 0)
+		return ERR_PTR(err);
+
+	return counter;
+}
+EXPORT_SYMBOL_GPL(devm_counter_alloc);
+
+/**
+ * devm_counter_add - complete registration of a counter
+ * @dev: the device to register the release callback for
+ * @counter: the counter to add
+ *
+ * This is the device managed version of counter_add(). It registers a cleanup
+ * callback to care for calling counter_unregister().
+ */
+int devm_counter_add(struct device *dev,
+		     struct counter_device *const counter)
+{
+	int err;
+
+	err = counter_add(counter);
+	if (err < 0)
+		return err;
+
+	return devm_add_action_or_reset(dev, devm_counter_release, counter);
+}
+EXPORT_SYMBOL_GPL(devm_counter_add);
+
 #define COUNTER_DEV_MAX 256
 
 static int __init counter_init(void)

include/linux/counter.h

@@ -327,14 +327,29 @@ struct counter_device {
 	spinlock_t events_in_lock;
 	struct mutex events_out_lock;
 	struct mutex ops_exist_lock;
+
+	/*
+	 * This can go away once all drivers are converted to
+	 * counter_alloc()/counter_add().
+	 */
+	bool legacy_device;
 };
 
 void *counter_priv(const struct counter_device *const counter);
 
 int counter_register(struct counter_device *const counter);
+
+struct counter_device *counter_alloc(size_t sizeof_priv);
+void counter_put(struct counter_device *const counter);
+int counter_add(struct counter_device *const counter);
+
 void counter_unregister(struct counter_device *const counter);
 int devm_counter_register(struct device *dev,
 			  struct counter_device *const counter);
+struct counter_device *devm_counter_alloc(struct device *dev,
+					  size_t sizeof_priv);
+int devm_counter_add(struct device *dev,
+		     struct counter_device *const counter);
 void counter_push_event(struct counter_device *const counter, const u8 event,
 			const u8 channel);