[PATCH 5.6 018/167] nvme: fix deadlock caused by ANA update wrong locking

[Greg Kroah-Hartman]

From: Sagi Grimberg <sagi@xxxxxxxxxxx>

[ Upstream commit 657f1975e9d9c880fa13030e88ba6cc84964f1db ]

The deadlock combines 4 flows in parallel:
- ns scanning (triggered from reconnect)
- request timeout
- ANA update (triggered from reconnect)
- I/O coming into the mpath device

(1) ns scanning triggers disk revalidation -> update disk info ->
    freeze queue -> but blocked, due to (2)

(2) timeout handler reference the g_usage_counter - > but blocks in
    the transport .timeout() handler, due to (3)

(3) the transport timeout handler (indirectly) calls nvme_stop_queue() ->
    which takes the (down_read) namespaces_rwsem - > but blocks, due to (4)

(4) ANA update takes the (down_write) namespaces_rwsem -> calls
    nvme_mpath_set_live() -> which synchronize the ns_head srcu
    (see commit 504db087aacc) -> but blocks, due to (5)

(5) I/O came into nvme_mpath_make_request -> took srcu_read_lock ->
    direct_make_request > blk_queue_enter -> but blocked, due to (1)

==> the request queue is under freeze -> deadlock.

The fix is making ANA update take a read lock as the namespaces list
is not manipulated, it is just the ns and ns->head that are being
updated (which is protected with the ns->head lock).

Fixes: 0d0b660f214dc ("nvme: add ANA support")
Signed-off-by: Sagi Grimberg <sagi@xxxxxxxxxxx>
Reviewed-by: Keith Busch <kbusch@xxxxxxxxxx>
Reviewed-by: Hannes Reinecke <hare@xxxxxxx>
Signed-off-by: Christoph Hellwig <hch@xxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 drivers/nvme/host/multipath.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
index a11900cf3a365..906dc0faa48ec 100644
--- a/drivers/nvme/host/multipath.c
+++ b/drivers/nvme/host/multipath.c
@@ -514,7 +514,7 @@ static int nvme_update_ana_state(struct nvme_ctrl *ctrl,
 	if (!nr_nsids)
 		return 0;
 
-	down_write(&ctrl->namespaces_rwsem);
+	down_read(&ctrl->namespaces_rwsem);
 	list_for_each_entry(ns, &ctrl->namespaces, list) {
 		unsigned nsid = le32_to_cpu(desc->nsids[n]);
 
@@ -525,7 +525,7 @@ static int nvme_update_ana_state(struct nvme_ctrl *ctrl,
 		if (++n == nr_nsids)
 			break;
 	}
-	up_write(&ctrl->namespaces_rwsem);
+	up_read(&ctrl->namespaces_rwsem);
 	return 0;
 }
 
-- 
2.20.1