Re: [PATCH 2/8] md: implement ->free_disk

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On 2022-07-13 01:17, Christoph Hellwig wrote:
> On Tue, Jul 12, 2022 at 05:13:48PM -0600, Logan Gunthorpe wrote:
>>> +
>>> +	percpu_ref_exit(&mddev->writes_pending);
>>> +	bioset_exit(&mddev->bio_set);
>>> +	bioset_exit(&mddev->sync_set);
>>> +
>>> +	kfree(mddev);
>>> +}
>>
>> I still don't think this is entirely correct. There are error paths that
>> will put the kobject before the disk is created and if they get hit then
>> the kfree(mddev) will never be called and the memory will be leaked.
> 
> True.
> 
>> Instead of creating an ugly special path for that, I came up with a solution 
>> that I think  makes a bit more sense: the kobject is still freed in it's 
>> own free  function, but the disk holds a reference to the kobject and drops
>> it in its free function. The sysfs puts and del_gendisk are then moved
>> into mddev_delayed_delete() so they happen earlier.
> 
> I'm not sure this is a good idea.  The mddev kobject hangs off the
> disk, so I don't think that it should in any way control the life
> time of the disk, as that just creates potential problems down the
> road.

My interpretation was that kobject_del() (which is called in
mddev_delayed_delete() before the disk would be removed) removes the
mddev from the disk. At that point, it's just a structure hanging around
that will be freed when its last reference is dropped, which is the disk
itself cleaning up. I'm not sure how that would cause potential problems.

Logan



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