bug in tag handling in blk-mq?

[Paolo Valente]

Hi Jens, Christoph, all,
Mike Galbraith has been experiencing hangs, on blk_mq_get_tag, only
with bfq [1].  Symptoms seem to clearly point to a problem in I/O-tag
handling, triggered by bfq because it limits the number of tags for
async and sync write requests (in bfq_limit_depth).

Fortunately, I just happened to find a way to apparently confirm it.
With the following one-liner for block/bfq-iosched.c:

@@ -554,8 +554,7 @@ static void bfq_limit_depth(unsigned int op, struct blk_mq_alloc_data *data)
        if (unlikely(bfqd->sb_shift != bt->sb.shift))
                bfq_update_depths(bfqd, bt);
 
-       data->shallow_depth =
-               bfqd->word_depths[!!bfqd->wr_busy_queues][op_is_sync(op)];
+       data->shallow_depth = 1;
 
        bfq_log(bfqd, "[%s] wr_busy %d sync %d depth %u",
                        __func__, bfqd->wr_busy_queues, op_is_sync(op),

Mike's machine now crashes soon and systematically, while nothing bad
happens on my machines, even with heavy workloads (apart from an
expected throughput drop).

This change simply reduces to 1 the maximum possible value for the sum
of the number of async requests and of sync write requests.

This email is basically a request for help to knowledgeable people.  To
start, here are my first doubts/questions:
1) Just to be certain, I guess it is not normal that blk-mq hangs if
async requests and sync write requests can be at most one, right?
2) Do you have any hint to where I could look for, to chase this bug?
Of course, the bug may be in bfq, i.e, it may be a somehow unrelated
bfq bug that causes this hang in blk-mq, indirectly.  But it is hard
for me to understand how.

Looking forward to some help.

Thanks,
Paolo

[1] https://www.spinics.net/lists/stable/msg215036.html