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

[Mike Snitzer]

On Tue, Nov 15 2016 at  8:08pm -0500,
Bart Van Assche <bart.vanassche at sandisk.com> wrote:

> On 11/15/2016 05:01 PM, Mike Snitzer wrote:
> >On Tue, Nov 15 2016 at  7:40pm -0500,
> >Bart Van Assche <bart.vanassche at sandisk.com> wrote:
> >>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?
> >
> >That shouldn't matter.  Once the type is established, it is used to
> >initialize the DM device's request_queue, the type cannot change across
> >different table loads.
> 
> For a single queue dm device, what prevents a user from removing all
> single queue paths and to add one or more blk-mq paths? I think this
> will cause dm_table_determine_type() to change the table type.

A new table is created for every table load (any time a multipath device
is loaded into the kernel).  The DM core disallows a table with a
different type to load.  It will be rejected within an error.

See dm-ioctl.c:table_load()