linux-kernel - blk_mq_update_queue_map makes an (invalid?) assumption about cpu ordering

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Message-ID: <x49fvs2pgjx.fsf@segfault.boston.devel.redhat.com>
Date:	Tue, 15 Oct 2013 13:52:50 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	jaxboe@...ionio.com
Cc:	linux-kernel@...r.kernel.org
Subject: blk_mq_update_queue_map makes an (invalid?) assumption about cpu ordering

Hi, Jens,

blk_mq_update_queue_map does this:

        for_each_possible_cpu(i) {
                if (!cpu_online(i)) {
                        map[i] = 0;
                        continue;
                }

...
                first_sibling = get_first_sibling(i);
                if (first_sibling == i) {
                        map[i] = cpu_to_queue_index(nr_uniq_cpus, nr_queues,
                                                        queue);
                        queue++;
                } else
                        map[i] = map[first_sibling];

This assumes that the first_sibling is listed before any other siblings,
which I don't believe is true.  I don't think you get any guaranteed
ordering in that cpu_possible_mask.

... or did I miss something?

Cheers,
Jeff
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