Re: [RFC PATCH 2/2] dm: only initialize full request_queue for request-baseddevice

From: Kiyoshi Ueda
Date: Fri May 21 2010 - 04:36:18 EST

Hi Mike,

On 05/21/2010 02:07 AM +0900, Mike Snitzer wrote:
> Kiyoshi Ueda <k-ueda@xxxxxxxxxxxxx> wrote:
>> On 05/19/2010 11:39 PM +0900, Mike Snitzer wrote:
>>> Thanks again for pointing this out; I'll work to arrive at an
>>> alternative locking scheme. Likely introduce a lock local to the
>>> multiple_device (effectively the 'queue_lock' I had before). But
>>> difference is I'd take that lock before taking _hash_lock.
>> You have to see do_resume(), too.
>> do_resume() gets hc->new_map with _hash_lock held but without your
>> 'queue_lock' held. This could cause inconsistent status;
>> table_load() for bio-based table do_resume() for rq-based table
>> --------------------------------------------------------------------
>> dm_lock_md_queue(md)
>> dm_table_setup_md_queue(t) (<= bio-based)
>> dm_clear_request_based_queue(md)
>> md->queue->request_fn = NULL
>> down_write(_hash_lock)
>> new_map = hc->new_map (<= rq-based)
>> up_write(_hash_lock)
>> dm_swap_table(md, new_map)
>> __bind(md, new_map) (<= rq-based)
>> down_write(_hash_lock)
>> hc->new_map = t
>> up_write(_hash_lock)
>> dm_unlock_md_queue(md)
>>>>> Also, your patch changes the queue configuration even when a table is
>>>>> already active and used. (e.g. Loading bio-based table to a mapped_device
>>>>> which is already active/used as request-based sets q->requst_fn in NULL.)
>>>>> That could cause some critical problems.
>>>> Yes, that is possible and I can add additional checks to prevent this.
>>>> But this speaks to a more general problem with the existing DM code.
>>>> dm_swap_table() has the negative check to prevent such table loads, e.g.:
>>>> /* cannot change the device type, once a table is bound */
>>>> This check should come during table_load, as part of
>>>> dm_table_set_type(), rather than during table resume.
>>> I'll look to address this issue in v6 too.
>> It can race between table_load() and do_resume() as well;
>> table_load() for bio-based do_resume() for rq-based
>> ---------------------------------------------------------------------
>> dm_table_set_type(t) (<= bio-based)
>> live_table = dm_get_live_table(md)
>> (assume no live_table yet)
>> new_map = hc->new_map (<= rq-based)
>> dm_swap_table(md, new_map)
>> __bind(md, new_map)
>> dm_table_setup_md_queue(t)
>> dm_clear_request_based_queue(md)
>> md->queue->request_fn = NULL
> Understood.
> I've addressed the above races by:
> 1) properly protecting md->queue during do_resume(). Both do_resume()
> and table_load() first take md->queue_lock and then _hash_lock.
> 2) adding a negative check for an existing live table to
> dm_table_setup_md_queue().
> I will mail out v7 after I've given it additional review.

dm_table_setup_md_queue() may allocate memory with blocking mode
inside md->queue_lock which is needed to resume the md.
That could cause a deadlock which doesn't happen in the current model;
- Assume a bio-based table has been already loaded in hc->new_map
and no live table yet in a mapped_device.
- Load a request-based table and then no memory situation happens
in dm_table_setup_md_queue().
- Then, the mapped_device can't be resumed even in the case that
resuming the mapped_device with the preloaded bio-based table
can make some memory by writeback.
=> deadlock

Kiyoshi Ueda
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