Re: blktests failures with v6.4

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#3: nvme/003 (fabrics transport)

     When nvme test group is run with trtype=rdma or tcp, the test case fails
     due to lockdep WARNING "possible circular locking dependency detected".
     Reported in May/2023. Bart suggested a fix for trytpe=rdma [4] but it
     needs more discussion.

     [4] https://lore.kernel.org/linux-nvme/20230511150321.103172-1-bvanassche@xxxxxxx/

This patch is unfortunately incorrect and buggy.

This will likely make the issue go away, but adds another
old issue where a client can DDOS a target by bombarding it
with connect/disconnect. When releases are async and we don't
have any back-pressure, it is likely to happen.
--
diff --git a/drivers/nvme/target/rdma.c b/drivers/nvme/target/rdma.c
index 4597bca43a6d..8b4f4aa48206 100644
--- a/drivers/nvme/target/rdma.c
+++ b/drivers/nvme/target/rdma.c
@@ -1582,11 +1582,6 @@ static int nvmet_rdma_queue_connect(struct rdma_cm_id
*cm_id,
                 goto put_device;
         }

-       if (queue->host_qid == 0) {
-               /* Let inflight controller teardown complete */
-               flush_workqueue(nvmet_wq);
-       }
-
         ret = nvmet_rdma_cm_accept(cm_id, queue, &event->param.conn);
         if (ret) {
                 /*
diff --git a/drivers/nvme/target/tcp.c b/drivers/nvme/target/tcp.c
index 868aa4de2e4c..c8cfa19e11c7 100644
--- a/drivers/nvme/target/tcp.c
+++ b/drivers/nvme/target/tcp.c
@@ -1844,11 +1844,6 @@ static u16 nvmet_tcp_install_queue(struct nvmet_sq
*sq)
         struct nvmet_tcp_queue *queue =
                 container_of(sq, struct nvmet_tcp_queue, nvme_sq);

-       if (sq->qid == 0) {
-               /* Let inflight controller teardown complete */
-               flush_workqueue(nvmet_wq);
-       }
-
         queue->nr_cmds = sq->size * 2;
         if (nvmet_tcp_alloc_cmds(queue))
                 return NVME_SC_INTERNAL;
--

Thanks Sagi, I tried the patch above and confirmed the lockdep WARN disappears
for both rdma and tcp. It indicates that the flush_workqueue(nvmet_wq)
introduced the circular lock dependency.

Thanks for confirming. This was expected.

I also found the two commits below
record why the flush_workqueue(nvmet_wq) was introduced.

  777dc82395de ("nvmet-rdma: occasionally flush ongoing controller teardown")
  8832cf922151 ("nvmet: use a private workqueue instead of the system workqueue")

The second patch is unrelated, before we used a global workqueue and
fundamentally had the same issue.

The left question is how to avoid both the connect/disconnect bombarding DDOS
and the circular lock possibility related to the nvmet_wq completion.

I don't see any way to synchronize connects with releases without moving connect sequences to a dedicated thread. Which in my mind is undesirable.

The only solution I can think of is to fail a host connect expecting the
host to reconnect and throttle this way, but that would lead to spurious
connect failures (at least from the host PoV).

Maybe we can add a NOT_READY connect error code in nvme for that...




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