Re: [PATCH] Fix blkback/blktap sysfs read bug.

["Joe Jin" <joe.jin@oracle.com>]

On 2010-01-20 11:06, Jan Beulich wrote:
> >>> "Joe Jin" <joe.jin@oracle.com> 20.01.10 11:51 >>>
> >On 2010-01-20 07:46, Jan Beulich wrote:
> >> And btw., in both cases with the lock added there's no need to check
> >> both 'be' and 'be->blkif', since be->blkif can't be NULL when be is
> >> non-NULL.
> >
> >No we must check it, as I have methioned, root cause is vbd was been
> >freed after open sysfs file.
> 
> Why? You're inside the locked region, so either 'be' is NULL, or both
> 'be' and 'be->blkif' are non-NULL.
> 

Sorry for misunstdrstood, your are right, attach new patch.


diff -r 0bec29c94ce9 drivers/xen/blkback/xenbus.c
--- a/drivers/xen/blkback/xenbus.c	Mon Jan 18 14:50:43 2010 +0000
+++ b/drivers/xen/blkback/xenbus.c	Wed Jan 20 19:44:32 2010 +0800
@@ -26,6 +26,8 @@
 #define DPRINTK(fmt, args...)				\
 	pr_debug("blkback/xenbus (%s:%d) " fmt ".\n",	\
 		 __FUNCTION__, __LINE__, ##args)
+
+static rwlock_t sysfs_read_lock = RW_LOCK_UNLOCKED;
 
 struct backend_info
 {
@@ -104,10 +106,19 @@
 				   struct device_attribute *attr,	\
 				   char *buf)				\
 	{								\
-		struct xenbus_device *dev = to_xenbus_device(_dev);	\
-		struct backend_info *be = dev->dev.driver_data;		\
+		ssize_t ret = -ENODEV;					\
+		struct xenbus_device *dev;				\
+		struct backend_info *be;				\
 									\
-		return sprintf(buf, format, ##args);			\
+		if (!get_device(_dev))					\
+			return ret;					\
+		dev = to_xenbus_device(_dev);				\
+		read_lock(&sysfs_read_lock);				\
+		if ((be = dev->dev.driver_data) != NULL)		\
+			ret = sprintf(buf, format, ##args);		\
+		read_unlock(&sysfs_read_lock);				\
+		put_device(_dev);					\
+		return ret;						\
 	}								\
 	static DEVICE_ATTR(name, S_IRUGO, show_##name, NULL)
 
@@ -173,6 +184,7 @@
 
 	DPRINTK("");
 
+	write_lock(&sysfs_read_lock);
 	if (be->major || be->minor)
 		xenvbd_sysfs_delif(dev);
 
@@ -191,6 +203,7 @@
 
 	kfree(be);
 	dev->dev.driver_data = NULL;
+	write_unlock(&sysfs_read_lock);
 	return 0;
 }
 
diff -r 0bec29c94ce9 drivers/xen/blktap/xenbus.c
--- a/drivers/xen/blktap/xenbus.c	Mon Jan 18 14:50:43 2010 +0000
+++ b/drivers/xen/blktap/xenbus.c	Wed Jan 20 19:44:32 2010 +0800
@@ -50,6 +50,7 @@
 	int group_added;
 };
 
+static rwlock_t sysfs_read_lock = RW_LOCK_UNLOCKED;
 
 static void connect(struct backend_info *);
 static int connect_ring(struct backend_info *);
@@ -122,10 +123,19 @@
 				   struct device_attribute *attr,	\
 				   char *buf)				\
 	{								\
-		struct xenbus_device *dev = to_xenbus_device(_dev);	\
-		struct backend_info *be = dev->dev.driver_data;		\
+		ssize_t ret = -ENODEV;					\
+		struct xenbus_device *dev;				\
+		struct backend_info *be;				\
 									\
-		return sprintf(buf, format, ##args);			\
+		if (!get_device(_dev))					\
+			return ret;					\
+		dev = to_xenbus_device(_dev);				\
+		read_lock(&sysfs_read_lock);				\
+		if ((be = dev->dev.driver_data) != NULL)		\
+			ret = sprintf(buf, format, ##args);		\
+		read_unlock(&sysfs_read_lock);				\
+		put_device(_dev);					\
+		return ret;						\
 	}								\
 	static DEVICE_ATTR(name, S_IRUGO, show_##name, NULL)
 
@@ -170,6 +180,7 @@
 {
 	struct backend_info *be = dev->dev.driver_data;
 
+	write_lock(&sysfs_read_lock);
 	if (be->group_added)
 		xentap_sysfs_delif(be->dev);
 	if (be->backend_watch.node) {
@@ -187,6 +198,7 @@
 	}
 	kfree(be);
 	dev->dev.driver_data = NULL;
+	write_unlock(&sysfs_read_lock);
 	return 0;
 }