[Xen-devel] [RFC] [PATCH] xen/blkback: add locking in statistics sysfs show functions

[Matt Wilson <msw@xxxxxxxxxx>]

This is a port of a patch in the XenoLinux 2.6.18 tree:
http://xenbits.xen.org/hg/linux-2.6.18-xen.hg/rev/f47c07325a56

Fix blkback sysfs race

Read blkback statistics info after remove vbd device(s) kernel
will crash.

Signed-off-by: Joe Jin <joe.jin@xxxxxxxxxx>
Acked-by: Jan Beulich <jbeulich@xxxxxxxxxx>
[Ported to Linux 3.x]
Signed-off-by: Matt Wilson <msw@xxxxxxxxxx>
---
 drivers/block/xen-blkback/xenbus.c |   20 +++++++++++++++++---
 1 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/drivers/block/xen-blkback/xenbus.c 
b/drivers/block/xen-blkback/xenbus.c
index da19506..74d4810 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -21,6 +21,8 @@
 #include <xen/grant_table.h>
 #include "common.h"
 
+static DEFINE_RWLOCK(sysfs_read_lock);
+
 struct backend_info {
        struct xenbus_device    *dev;
        struct xen_blkif        *blkif;
@@ -225,10 +227,20 @@ int __init xen_blkif_interface_init(void)
                                   struct device_attribute *attr,       \
                                   char *buf)                           \
        {                                                               \
-               struct xenbus_device *dev = to_xenbus_device(_dev);     \
-               struct backend_info *be = dev_get_drvdata(&dev->dev);   \
+               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);                            \
+               be = (struct backend_info *) dev_get_drvdata(&dev->dev);\
+               if (be != 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)
 
@@ -353,6 +365,7 @@ static int xen_blkbk_remove(struct xenbus_device *dev)
 
        DPRINTK("");
 
+       write_lock(&sysfs_read_lock);
        if (be->major || be->minor)
                xenvbd_sysfs_delif(dev);
 
@@ -371,6 +384,7 @@ static int xen_blkbk_remove(struct xenbus_device *dev)
 
        kfree(be);
        dev_set_drvdata(&dev->dev, NULL);
+       write_unlock(&sysfs_read_lock);
        return 0;
 }
 
-- 
1.7.2.5


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