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Re: [dm-devel] Crash in dm_done()

On 01/11/2012 05:25 PM, Mike Snitzer wrote:
> On Wed, Jan 11 2012 at  3:36am -0500,
> Jun'ichi Nomura <j-nomura ce jp nec com> wrote:
>> Hi Hannes,
>> On 01/10/12 20:18, Hannes Reinecke wrote:
>>> I'm trying to hunt down a mysterious crash:
>>> Unable to handle kernel pointer dereference at virtual kernel
>>> address 000003c001762000
>>> Oops: 0011 [#1] SMP
>>> Modules linked in: dm_round_robin sg sd_mod crc_t10dif ipv6 loop
>>> dm_multipath scsi_dh dm_mod qeth_l3 ipv6_lib zfcp scsi_transport_fc
>>> scsi_tgt scsi_mod dasd_eckd_mod dasd_mod qeth qdio ccwgroup ext3
>>> mbcache jbd
>>> Supported: Yes
>>> CPU: 0 Not tainted 3.0.13-0.5-default #1
>>> Process kworker/0:1 (pid: 51750, task: 0000000008450138, ksp:
>>> 0000000007323620)
>>> Krnl PSW : 0704200180000000 000003c001953750 (dm_done+0x44/0x188
>>> [dm_mod])
>>>            R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:0 CC:2 PM:0 EA:3
>>> Krnl GPRS: 000000002adb5e80 000003c001762100 0000000037de9e80
>>> 0000000000000000
>>>            0000000000000400 000003c00195e178 0000000000000380
>>> 0000000000000000
>>>            0000000000000100 0000000000000001 0000000037de9e80
>>> 0000000037de9e68
>>>            000003c00194f000 000003c00195e168 000000007ef97d30
>>> 000000007ef97cd8
>>> Krnl Code: 000003c001953740: e3b021580004       lg     %r11,344(%r2)
>>>            000003c001953746: e310b0080004       lg     %r1,8(%r11)
>>>            000003c00195374c: bf4fb180           icm %r4,15,384(%r11)
>>>           >000003c001953750: e32010080004       lg      %r2,8(%r1)
>>>            000003c001953756: e38020500004       lg      %r8,80(%r2)
>>>            000003c00195375c: a7740062           brc    7,3c001953820
>>>            000003c001953760: b9020099           ltgr    %r9,%r9
>>>            000003c001953764: a7840049           brc    8,3c0019537f6
>>> r11 is struct dm_rq_target_io *tio = clone->end_io_data;
>>> r1 is tio->ti (ie struct dm_target), which is invalid.
>>> r2 is tio->ti->type, likewise.
>>> Apparently the table got replaced between map_io() and dm_done(),
>>> causing this invalid pointer.
>>> While we do hold a reference on the mapped_device in map_request(),
>>> we only take a _single_ reference to the table in dm_request_fn(),
>>> which is dropped again at the end.
>>> And as the table holds the pointer to the targets, they'll be
>>> invalidated upon table swapping, causing dm_done() accessing an
>>> invalid pointer.
>> The last paragraph is not correct.
>> If any requests are in-flight, dm does not swap table.
>> I.e., in dm_suspend(), for request-based dm, we do:
>>   1) stop request_queue processing
>>      <from this point, no new request becomes in-flight>
>>   2) wait for completion of in-flight I/Os
>>      <from this point, no requests are in-flight>
>> and only after that, we can swap tables.
>> Existence of in-flight I/O is checked by "pending" counter of md.
>> The counter is increased in dm_request_fn()
>> and decreased in rq_completed(), which is called either
>> when the original request is requeued or completed.
>> I.e. after dm_done() processing.
>>> I can't really believe that is the case, so please do correct me if
>>> the above analysis is wrong.
>> Just guessing...
>> Such a mysterious crash could occur if there are bugs like:
>>   - somebody start the queue while dm stopped it in dm_suspend()
>>   - somebody submit/complete/requeue request with
>>     wrong function and corrupt pending counter
>>   - lower-level driver completes a request twice
>> If you can recreate the crash, try attached debug patch.
>> It should raise warnings when cases like above happen
>> and might help hunting down the problem.
> There was also that earlier thread about a crash in dm_done(), where
> dereferencing clone->end_io_data caused the crash:
> https://lkml.org/lkml/2011/10/31/97
> Heiko said that Hannes' patch:
> http://www.redhat.com/archives/dm-devel/2011-November/msg00176.html
> ... actually helped:
> https://lkml.org/lkml/2011/11/29/168
> But we never did get to the bottom of _why_, and Jun'ichi pointed out
> a NULL pointer check is missing:
> http://www.redhat.com/archives/dm-devel/2011-December/msg00022.html
> Anyway, Hannes, if you can reproduce it'd also be interesting to see if
> your patch (updated with NULL pointer check?) makes any difference.
Sad news: This is _with_ my patch applied.
(Hey, these are my patches. Obviously I will be using them :-)

But then, there have been about 30 DM ioctls running at the same
time (thanks to udev), so chances are we're hitting an awkward race


Dr. Hannes Reinecke		      zSeries & Storage
hare suse de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)

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