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

["Joe Jin" <joe.jin@xxxxxxxxxx>]

On 2010-01-20 11:06, Jan Beulich wrote:
> >>> "Joe Jin" <joe.jin@xxxxxxxxxx> 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;
 }
 

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