The gpiommc configfs context structure needs locking, as configfs
does not lock access between files.

--- a/drivers/mmc/host/gpiommc.c
+++ b/drivers/mmc/host/gpiommc.c
@@ -143,6 +143,8 @@
 	struct platform_device *pdev;
 	/* The configuration */
 	struct gpiommc_platform_data pdata;
+	/* Mutex to protect this structure */
+	struct mutex mutex;
 };
 
 #define GPIO_INVALID	-1
@@ -233,6 +235,8 @@
 	unsigned int gpio;
 	int err = 0;
 
+	mutex_lock(&dev->mutex);
+
 	if (attr == &gpiommc_attr_DI) {
 		gpio = dev->pdata.pins.gpio_di;
 		if (gpio == GPIO_INVALID)
@@ -293,6 +297,8 @@
 	WARN_ON(1);
 	err = -ENOSYS;
 out:
+	mutex_unlock(&dev->mutex);
+
 	return err ? err : count;
 }
 
@@ -352,6 +358,8 @@
 	int err = -EINVAL;
 	unsigned long data;
 
+	mutex_lock(&dev->mutex);
+
 	if (attr == &gpiommc_attr_register) {
 		err = strict_strtoul(page, 10, &data);
 		if (err)
@@ -477,6 +485,8 @@
 	WARN_ON(1);
 	err = -ENOSYS;
 out:
+	mutex_unlock(&dev->mutex);
+
 	return err ? err : count;
 }
 
@@ -513,6 +523,7 @@
 	if (!dev)
 		return NULL;
 
+	mutex_init(&dev->mutex);
 	config_item_init_type_name(&dev->item, name,
 				   &gpiommc_dev_ci_type);
 
