linux-kernel - Re: 2.6.28-rc2: (mtd)block/partitions BUG with kobject reference count

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Date:	Fri, 31 Oct 2008 07:26:52 +0100
From:	"Kay Sievers" <kay.sievers@...y.org>
To:	"Bryan Wu" <cooloney@...nel.org>
Cc:	"Peter Korsgaard" <jacmet@...site.dk>,
	"Rafael J. Wysocki" <rjw@...k.pl>, linux-mtd@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: 2.6.28-rc2: (mtd)block/partitions BUG with kobject reference count

On Fri, Oct 31, 2008 at 05:24, Bryan Wu <cooloney@...nel.org> wrote:
> On Fri, Oct 31, 2008 at 7:47 AM, Kay Sievers <kay.sievers@...y.org> wrote:
>> On Thu, Oct 30, 2008 at 22:51, Kay Sievers <kay.sievers@...y.org> wrote:
>>> On Thu, Oct 30, 2008 at 00:28, Peter Korsgaard <jacmet@...site.dk> wrote:
>>>>>>>>> "Rafael" == Rafael J Wysocki <rjw@...k.pl> writes:
>>>>
>>>>  Rafael> On Wednesday, 29 of October 2008, Peter Korsgaard wrote:
>>>>  >> Hi,
>>>>  >>
>>>>  >> I'm seing what looks like a kobject reference count issue with
>>>>  >> mtdblock_ro + mtd_dataflash + mtd partitions and repeated unbind/bind.
>>>>  >> I'm on 2.6.28-rc2, but I can reproduce the problem on 2.6.27 as well.
>>>>
>>>>  Rafael> Is it reproducible with 2.6.26 too?
>>>>
>>>> Sorry, I haven't backported my platform support code to such "old"
>>>> kernel. I can do it though, if you think it will help pinpoint the
>>>> issue.
>>>
>>> This sounds like a possible reason for the problem:
>>> "After digging into the mtd code, this bug is not related to our driver. It
>>> should be a subtle bug in mtd core code.
>>>
>>> In add_mtd_partition, for 2 partitions, 2 gendisk structures will be
>>> allocated. But these 2 gendisk->queue will be set to the same
>>> request_queue. Then when unregistering the 1st partition, from the
>>> same request_queue->backing_dev_info, the bdi struct will be set to
>>> NULL. So for the 2nd partition (bdi == NULL), the sysfs dir of 2nd
>>> partition will not be removed. Finally, when modprobe the module
>>> again, the 2nd partition won't be added"
>>> https://blackfin.uclinux.org/gf/tracker/4463
>>
>> Looks like a bdi issue:
>>  http://lkml.org/lkml/2008/10/30/519
>>
>> Peter, if I do this (whitespace mangled, just pasted in here), the
>> error goes away for me. Can you try this?

>> --- a/block/genhd.c
>> +++ b/block/genhd.c
>> @@ -515,7 +515,8 @@ void add_disk(struct gendisk *disk)
>>        blk_register_queue(disk);
>>
>>        bdi = &disk->queue->backing_dev_info;
>> -       bdi_register_dev(bdi, disk_devt(disk));
>> +       if (!bdi->dev)
>> +               bdi_register_dev(bdi, disk_devt(disk));
>>        retval = sysfs_create_link(&disk_to_dev(disk)->kobj, &bdi->dev->kobj,
>>                                   "bdi");
>>        WARN_ON(retval);
>
> IMHO, this is a workaround, right? I think the final solution should
> provide every 'gendisk' a dedicated 'bdi'.
> So they won't mess up and overwrite.

It's a per-queue property which is shared by multiple devices. The
"bdi" symlink of the blockdev still points to the actual (shared) bdi
device. It's just that only the first device, of the pool of devices
sharing a queue, creates the bdi entry.

Not sure what the final fix should be, but this sounds better than
duplicating identical information and/or leaking the duplicates. :)

Kay
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