Re: kobject: provide kobject_put_wait to fix module unload race

[Mike Snitzer <snitzer@redhat.com>]

On Mon, Jan 06 2014 at  1:55pm -0500,
Mikulas Patocka <mpatocka@redhat.com> wrote:

> 
> 
> On Sun, 5 Jan 2014, Greg Kroah-Hartman wrote:
> 
> > On Sun, Jan 05, 2014 at 05:43:56PM +0100, Bart Van Assche wrote:
> > > On 01/04/14 19:06, Mikulas Patocka wrote:
> > > > -	if (t && !t->release)
> > > > -		pr_debug("kobject: '%s' (%p): does not have a release() "
> > > > -			 "function, it is broken and must be fixed.\n",
> > > > -			 kobject_name(kobj), kobj);
> > > > -
> > > 
> > > Has it been considered to issue a warning if no release function has
> > > been defined and free_completion == NULL instead of removing the above
> > > debug message entirely ? I think even with this patch applied it is
> > > still wrong to invoke kobject_put() on an object without defining a
> > > release function.
> > 
> > This patch isn't going to be applied, and I've reverted the original
> > commit, so there shouldn't be any issues anymore with this code.
> 
> Why? This patch does the same thing as 
> eee031649707db3c9920d9498f8d03819b74fc23, but it's smaller. So why did you 
> accept eee031649707db3c9920d9498f8d03819b74fc23 and not this?
> 
> The code to wait for kobject destruction using completion already exists 
> in cpufreq_sysfs_release, cpuidle_sysfs_release, 
> cpuidle_state_sysfs_release, cpuidle_driver_sysfs_release, 
> ext4_sb_release, ext4_feat_release, f2fs_sb_release (these are the only 
> kobject users that are correct w.r.t. module unloading), so if you accept 
> this patch, you can simplify them to use kobject_put_wait.

Hi Mikulas,

Please just submit a DM-only patch that follows the same racey pattern
of firing a completion from the kobj_type .release method in dm_mod.
I'll get it queued up for 3.14.

If/when we gets reports of a crash due to dm_mod unload racing with
kobject_put we can revisit this.

Thanks,
Mike