[dm-devel] [PATCH 4/7] dm mpath: remove process_queued_ios()
Hannes Reinecke
hare at suse.de
Tue Feb 4 09:45:14 UTC 2014
On 02/04/2014 10:27 AM, Junichi Nomura wrote:
> On 02/04/14 18:08, Hannes Reinecke wrote:
>> On 02/04/2014 09:55 AM, Junichi Nomura wrote:
>>> On 02/04/14 17:18, Hannes Reinecke wrote:
>>>>>> @@ -1591,8 +1563,17 @@ static int multipath_ioctl(struct dm_target *ti, unsigned int cmd,
>>>>> ...
>>>>>> + if (m->current_pg && m->pg_init_required)
>>>>>> + __pg_init_all_paths(m, 0);
>>>>>> + spin_unlock_irqrestore(&m->lock, flags);
>>>>>> + dm_table_run_md_queue_async(m->ti->table);
>>>>>> + }
>>>>>
>>>>> What happens if "!m->current_pg && m->pg_init_required"?
>>>>>
>>>> >From the current logic it means that no valid pg was found, so
>>>> calling pg_init would be pointless.
>>>> We're calling __choose_pgpath() before that, so if that returns
>>>> with current_pg == NULL all paths are down, and calling
>>>> pg_init would be pointless.
>>>>
>>>> But I think I see to have pg_init_required handling cleared up;
>>>> I'll be doing a patch to unset it at the start of __choose_pgpath(),
>>>> this we we can be sure that it'll be set correctly.
>>>
>>> I think it is possible that __choose_pgpath() being called twice
>>> before pg_init_required is checked.
>>>
>>> For example,
>>>
>>> multipath_ioctl()
>>> __choose_pgpath()
>>> clear pg_init_required
>>> select a new pg
>>> __switch_pg()
>>> set pg_init_required
>>>
>>> map_io()
>>> __choose_pgpath()
>>> clear pg_init_required
>>> select the same pg
>>> (pg_init_required is not set)
>>> ...
>>>
>> But why should 'map_io' calling __choose_pgpath()?
>>
>> Either __choose_path() from ioctl was able to set ->current_pg
>> (in which case __choose_pgpath() wouldn't be called in map_io)
>> or it was not, in which case pg_init_required would need to be reset
>> during __choose_pgpath() when called from map_io().
>
> __choose_pgpath() is a function to select "path".
> Even if current_pg is set, map_io() may call the function
> to select a path. (Please look at the repeat_count check)
>
... But only if 'queue_io' is not set, ie no pg_init is running.
At which point 'pg_init_required' should be set to '0' anyway.
What I'm arguing here is an inconsistency in __choose_pg():
__choose_pg() might or might not set 'current_pg', but if
'current_pg' is not set the status of 'pg_init_required'
is undefined upon return.
This makes it (in my view) unnecessarily complicated to
check if we need to initialize the paths, as we have to
check for both, current_pg _and_ pg_init_required.
If it were _that_ easy we wouldn't have this discussion :-)
> So, I suggested the followings:
> Call __pg_init_all_paths() regardless of current_pg.
> __pg_init_all_paths() returns whether pg_init has started.
> If !current_pg, it returns 0.
> (https://www.redhat.com/archives/dm-devel/2014-February/msg00013.html)
>
> The semantics is clear:
> - if pg_init_required, call __pg_init_all_paths()
> - __pg_init_all_paths() might fail to start pg_init (= returns 0).
> Then caller should take some action or give up.
>
All right, will be modifying the patch.
Cheers,
Hannes
--
Dr. Hannes Reinecke zSeries & Storage
hare at 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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