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

[Kiyoshi Ueda <k-ueda@ct.jp.nec.com>]

Hi Mike,

On 05/18/2010 10:46 PM +0900, Mike Snitzer wrote:
> Kiyoshi Ueda <k-ueda@ct.jp.nec.com> wrote:
>> On 05/18/2010 02:27 AM +0900, Mike Snitzer wrote:
>>> Kiyoshi Ueda <k-ueda@ct.jp.nec.com> wrote:
>>>> As far as I understand, the current model of device-mapper is:
>>>>   - a table (precisely, a target) has various attributes,
>>>>     bio-based/request-based is one of such attributes
>>>>   - a table and its attributes are bound to the block device on resume
>>>> If we want to fix a problem, I think we should either work based on
>>>> this model or change the model.
>>>>
>>>> Your patch makes that loading table affects the block device, so you
>>>> are changing the model.
>>>>
>>>> If you change the model, it should be done carefully.
>>>> For example, the current model allows most of the table loading code
>>>> to run without exclusive lock on the device because it doesn't affect
>>>> the device itself.  If you change this model, table loading needs to
>>>> be serialized with appropriate locking.
>>>
>>> Nice catch, yes md->queue needs protection (see patch below).
>>
>> Not enough. (See drivers/md/dm-ioctl.c:table_load().)
>> Table load sequence is:
>>   1. populate table
>>   2. set the table to ->new_map of the hash_cell for the mapped_device
>>      in protection by _hash_lock.
>>
>> Since your fix only serializes the step 1, concurrent table loading
>> could end up with inconsistent status; e.g. request-based table is
>> bound to the mapped_device while the queue is initialized as bio-based.
>> With your new model, those 2 steps above must be atomic.
> 
> Ah, yes.. I looked at the possibility of serializing the entirety of
> table_load but determined that would be too excessive (would reduce
> parallelism of table_load).  But I clearly missed the fact that there
> could be a race to the _hash_lock protected critical section in
> table_load() -- leading to queue inconsistency.
> 
> I'll post v5 of the overall patch which will revert the mapped_device
> 'queue_lock' serialization that I proposed in v4.  v5 will contain
> the following patch to localize all table load related queue
> manipulation to the _hash_lock protected critical section in
> table_load().  So it sets the queue up _after_ the table's type is
> established with dm_table_set_type().

dm_table_setup_md_queue() may allocate memory with blocking mode.
Blocking allocation inside exclusive _hash_lock can cause deadlock;
e.g. when it has to wait for other dm devices to resume to free some
memory.

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.

Thanks,
Kiyoshi Ueda