[Xen-devel] Re: [PATCH] Fix blk{back,tap} sysfs race

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

While I would have wanted it to use DEFINE_RWLOCK(), it's probably
okay without. Or maybe you could adjust this as you check it in, Keir?

Acked-by: Jan Beulich <jbeulich@xxxxxxxxxx>

>>> "Joe Jin" <joe.jin@xxxxxxxxxx> 21.01.10 01:46 >>>
Hi, 

Read blk{back,tap} statistics info after remove vbd device(s) kernel will crash.
Below patch will fix the race, please review and apply.

Signed-off-by: Joe Jin <joe.jin@xxxxxxxxxx>
Cc:  Keir Fraser <keir.fraser@xxxxxxxxxx>
Cc: Jan Beulich <JBeulich@xxxxxxxxxx>

---
 blkback/xenbus.c |   19 ++++++++++++++++---
 blktap/xenbus.c  |   18 +++++++++++++++---
 2 files changed, 31 insertions(+), 6 deletions(-)

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      Thu Jan 21 08:38:58 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       Thu Jan 21 08:38:58 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;
 }
 


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