Re: kobject: provide kobject_put_wait to fix module unload race
From: Mike Snitzer
Date: Mon Jan 06 2014 - 16:31:30 EST
On Mon, Jan 06 2014 at 1:55pm -0500,
Mikulas Patocka <mpatocka@xxxxxxxxxx> 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.
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.
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