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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Fri, Apr 20, 2012 at 07:21:13PM +0000, Matt Wilson wrote:
> 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.

Looks pretty reasonable.

> 
> 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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