[dm-devel] [PATCH 7/7] dm-mpath: Fix a race condition in __multipath_map()

[Bart Van Assche]

On 11/15/2016 04:37 PM, Mike Snitzer wrote:
> On Tue, Nov 15 2016 at  6:35pm -0500,
> Bart Van Assche <bart.vanassche at sandisk.com> wrote:
>> If a single-queue dm device is stacked on top of multi-queue block
>> devices and map_tio_request() is called while there are no paths then
>> the request will be prepared for a single-queue path. If a path is
>> added after a request was prepared and before __multipath_map() is
>> called return DM_MAPIO_REQUEUE such that it gets unprepared and
>> reprepared as a blk-mq request.
>
> This patch makes little sense to me.  There isn't a scenario that I'm
> aware of that would allow the request_queue to transition between old
> .request_fn and new blk-mq.
>
> The dm-table code should prevent this.

Hello Mike,

Are you aware that dm_table_determine_type() sets "all_blk_mq" to false 
if there are no paths, even if the dm device is in blk-mq mode?

Bart.