Re: [PATCH 0/3 rfc] Fix nvme-tcp and nvme-rdma controller reset hangs

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Will it work if nvme mpath used request NOWAIT flag for its submit_bio()
call, and add the bio to the requeue_list if blk_queue_enter() fails? I
think that looks like another way to resolve the deadlock, but we need
the block layer to return a failed status to the original caller.

Yes, I think BLK_MQ_REQ_NOWAIT makes total sense here.

BTW, the specific hang reported is not blocking on tag allocation, but
rather than on blk_queue_enter blocking on a frozen queue.

dm-mpath also uses it for its request allocation for similar reasons.

That is the rq based dm, and I think it is because dm_mq_queue_rq is
non-blocking. Care to explain what is similar to nvme-mpath?

I don't see how bio based dm cares about any of this...

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But who would kick the requeue list? and that would make near-tag-exhaust performance stink...

The multipath code would have to kick the list.

When? Not following your thoughts...

You are suggesting that we call submit_bio that will fail, put it on the
requeue_list and then what? blindly kick the requeue list? try to see if
there is an alternate path and then kick the list? for every bio that
comes in?

We could also try to
split into two flags, one that affects blk_queue_enter and one that
affects the tag allocation.

If this is something that can work reliably then its better off, plus we
can probably kill the srcu as well. But I don't see how this would
work unfortunately.

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moving nvme_start_freeze from nvme_rdma_teardown_io_queues to nvme_rdma_configure_io_queues can fix it.
It can also avoid I/O hang long time if reconnection failed.

Can you explain how we'd still ensure that no new commands get queued
during teardown using that scheme?

quiescing the queue would prevent new submissions from coming down to
the driver, but I don't see how this move can help here...

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