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

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

On 2010-01-20 18:16, Daniel Stodden wrote:
> 
> Hey.
> 
> I think reverted a similar patch when moving past 2.6.18 a while ago,
> but I don't remember the first kernel versions affected right now.
> 
> On Wed, 2010-01-20 at 06:45 -0500, Joe Jin wrote:
> 
> >  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);                           \
> 
> I think this deadlocks here:
> > +           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)
> 
> .. and here:
> >             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;
> >  }
> 
> Sysfs may refcount these nodes, blocking xentap_sysfs_delif().

Would you like to give me some info about of blocking and deadlocks?

Checked xentap_sysfs_delif() -> sysfs_remove_group() and I did not found
blocking codes, maybe I lost something?

Thanks,
Joe

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