#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...
