[dm-devel] v3.15 dm-mpath regression: cable pull test causes I/O hang

[Hannes Reinecke]

On 07/03/2014 03:56 PM, Bart Van Assche wrote:
> On 07/03/14 00:02, Mike Snitzer wrote:
>> On Fri, Jun 27 2014 at  9:33am -0400,
>> Mike Snitzer <snitzer at redhat.com> wrote:
>>
>>> On Fri, Jun 27 2014 at  9:02am -0400,
>>> Bart Van Assche <bvanassche at acm.org> wrote:
>>>
>>>> Hello,
>>>>
>>>> While running a cable pull simulation test with dm_multipath on top of
>>>> the SRP initiator driver I noticed that after a few iterations I/O locks
>>>> up instead of dm_multipath processing the path failure properly (see also
>>>> below for a call trace). At least kernel versions 3.15 and 3.16-rc2 are
>>>> vulnerable. This issue does not occur with kernel 3.14. I have tried to
>>>> bisect this but gave up when I noticed that I/O locked up completely with
>>>> a kernel built from git commit ID e809917735ebf1b9a56c24e877ce0d320baee2ec
>>>> (dm mpath: push back requests instead of queueing). But with the bisect I
>>>> have been able to narrow down this issue to one of the patches in "Merge
>>>> tag 'dm-3.15-changes' of git://git.kernel.org/pub/scm/linux/kernel/git/
>>>> device-mapper/linux-dm". Does anyone have a suggestion how to analyze this
>>>> further or how to fix this ?
>>
>> I still don't have a _known_ fix for your issue but I reviewed commit
>> e809917735ebf1b9a56c24e877ce0d320baee2ec closer and identified what
>> looks to be a regression in logic for multipath_busy, it now calls
>> !pg_ready() instead of directly checking pg_init_in_progress.  I think
>> this is needed (Hannes, what do you think?):
>>
>> diff --git a/drivers/md/dm-mpath.c b/drivers/md/dm-mpath.c
>> index 3f6fd9d..561ead6 100644
>> --- a/drivers/md/dm-mpath.c
>> +++ b/drivers/md/dm-mpath.c
>> @@ -373,7 +373,7 @@ static int __must_push_back(struct multipath *m)
>>   		 dm_noflush_suspending(m->ti)));
>>   }
>>
>> -#define pg_ready(m) (!(m)->queue_io && !(m)->pg_init_required)
>> +#define pg_ready(m) (!(m)->queue_io && !(m)->pg_init_required && !(m)->pg_init_in_progress)
>>
>>   /*
>>    * Map cloned requests
>
> Hello Mike,
>
> Sorry but even with this patch applied and additionally with commit IDs
> 86d56134f1b6 ("kobject: Make support for uevent_helper optional") and
> bcccff93af35 ("kobject: don't block for each kobject_uevent") reverted
> my multipath test still hangs after a few iterations. I also reran the
> same test with kernel 3.14.3 and it is still running after 30 iterations.
>
Hmm. Would've been too easy.
Sigh.

Cheers,

Hannes

-- 
Dr. Hannes Reinecke		      zSeries & Storage
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