crash when connecting to targets using nr_io_queues < num cpus

[Steve Wise]

> > > Hey all,
> > >
> > > I'm testing smaller ioq sets with nvmf/rdma, and I see some issue.  If I
> connect
> > > with 2, 4, 6, 8, 10, 16, or 32  for nr_io_queues, everything is happy.  It
> > > seems, though, if I connect with a value of 12, or 28, or some other non
> power
> > > of two, I get intermittent crashes in __blk_mq_get_reserved_tag() at line
> 337
> > > when setting up a controller's IO queues.   I'm not sure exactly if this
is
> > > always non power of two, or something else, but it seems to never crash
with
> > > power of two values (could be a coincidence I guess).
> >
> > I think Ming sent a patch for this some time ago... Not sure what
> > happened with it though...
> 
> This?
> 
> http://lists.infradead.org/pipermail/linux-nvme/2016-June/004884.html

This is indeed the same problem.  I don't have the knoggin to propose a fix.
Sagi/Christoph, do you have any ideas on this?  I'm willing to take and idea
forward and test it out of you all have any clever ideas.  We should at least
prevent setting nr_io_queues to a value that will crash immediately when nvmf is
used...

Steve.