[PATCH 2/2] nvmet: Fix fatal_err_work deadlock

[jsmart2021@gmail.com (James Smart)]

Below is  a stack trace for an issue that was reported.

What's happening is that the nvmet layer had it's controller kato
timeout fire, which causes it to schedule its fatal error handler
via the fatal_err_work element. The error handler is invoked, which
calls the transport delete_ctrl() entry point, and as the transport
tears down the controller, nvmet_sq_destroy ends up doing the final
put on the ctlr causing it to enter its free routine. The ctlr free
routine does a cancel_work_sync() on fatal_err_work element, which
then does a flush_work and wait_for_completion. But, as the wait is
in the context of the work element being flushed, its in a catch-22
and the thread hangs.

[  326.903131] nvmet: ctrl 1 keep-alive timer (15 seconds) expired!
[  326.909832] nvmet: ctrl 1 fatal error occurred!
[  327.643100] lpfc 0000:04:00.0: 0:6313 NVMET Defer ctx release xri
x114 flg x2
[  494.582064] INFO: task kworker/0:2:243 blocked for more than 120
seconds.
[  494.589638]       Not tainted 4.14.0-rc1.James+ #1
[  494.594986] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs"
disables this message.
[  494.603718] kworker/0:2     D    0   243      2 0x80000000
[  494.609839] Workqueue: events nvmet_fatal_error_handler [nvmet]
[  494.616447] Call Trace:
[  494.619177]  __schedule+0x28d/0x890
[  494.623070]  schedule+0x36/0x80
[  494.626571]  schedule_timeout+0x1dd/0x300
[  494.631044]  ? dequeue_task_fair+0x592/0x840
[  494.635810]  ? pick_next_task_fair+0x23b/0x5c0
[  494.640756]  wait_for_completion+0x121/0x180
[  494.645521]  ? wake_up_q+0x80/0x80
[  494.649315]  flush_work+0x11d/0x1a0
[  494.653206]  ? wake_up_worker+0x30/0x30
[  494.657484]  __cancel_work_timer+0x10b/0x190
[  494.662249]  cancel_work_sync+0x10/0x20
[  494.666525]  nvmet_ctrl_put+0xa3/0x100 [nvmet]
[  494.671482]  nvmet_sq_:q+0x64/0xd0 [nvmet]
[  494.676540]  nvmet_fc_delete_target_queue+0x202/0x220 [nvmet_fc]
[  494.683245]  nvmet_fc_delete_target_assoc+0x6d/0xc0 [nvmet_fc]
[  494.689743]  nvmet_fc_delete_ctrl+0x137/0x1a0 [nvmet_fc]
[  494.695673]  nvmet_fatal_error_handler+0x30/0x40 [nvmet]
[  494.701589]  process_one_work+0x149/0x360
[  494.706064]  worker_thread+0x4d/0x3c0
[  494.710148]  kthread+0x109/0x140
[  494.713751]  ? rescuer_thread+0x380/0x380
[  494.718214]  ? kthread_park+0x60/0x60

Correct by creating a final free work element, which is scheduled
by the final ctrl put routine, so that when the cancel_work_sync
is called, it cannot be in the same work_q element context.

Signed-off-by: James Smart <james.smart at broadcom.com>
---
 drivers/nvme/target/core.c  | 26 ++++++++++++++++++--------
 drivers/nvme/target/nvmet.h |  1 +
 2 files changed, 19 insertions(+), 8 deletions(-)

diff --git a/drivers/nvme/target/core.c b/drivers/nvme/target/core.c
index 035a3919d095..7c133e11d3c1 100644
--- a/drivers/nvme/target/core.c
+++ b/drivers/nvme/target/core.c
@@ -847,17 +847,12 @@ u16 nvmet_alloc_ctrl(const char *subsysnqn, const char *hostnqn,
 	return status;
 }
 
-static void nvmet_ctrl_free(struct kref *ref)
+static void nvmet_ctrl_free_work(struct work_struct *work)
 {
-	struct nvmet_ctrl *ctrl = container_of(ref, struct nvmet_ctrl, ref);
+	struct nvmet_ctrl *ctrl =
+			container_of(work, struct nvmet_ctrl, free_work);
 	struct nvmet_subsys *subsys = ctrl->subsys;
 
-	nvmet_stop_keep_alive_timer(ctrl);
-
-	mutex_lock(&subsys->lock);
-	list_del(&ctrl->subsys_entry);
-	mutex_unlock(&subsys->lock);
-
 	flush_work(&ctrl->async_event_work);
 	cancel_work_sync(&ctrl->fatal_err_work);
 
@@ -869,6 +864,21 @@ static void nvmet_ctrl_free(struct kref *ref)
 	kfree(ctrl);
 }
 
+static void nvmet_ctrl_free(struct kref *ref)
+{
+	struct nvmet_ctrl *ctrl = container_of(ref, struct nvmet_ctrl, ref);
+	struct nvmet_subsys *subsys = ctrl->subsys;
+
+	nvmet_stop_keep_alive_timer(ctrl);
+
+	mutex_lock(&subsys->lock);
+	list_del(&ctrl->subsys_entry);
+	mutex_unlock(&subsys->lock);
+
+	INIT_WORK(&ctrl->free_work, nvmet_ctrl_free_work);
+	schedule_work(&ctrl->free_work);
+}
+
 void nvmet_ctrl_put(struct nvmet_ctrl *ctrl)
 {
 	kref_put(&ctrl->ref, nvmet_ctrl_free);
diff --git a/drivers/nvme/target/nvmet.h b/drivers/nvme/target/nvmet.h
index 6c3a5a5fc71e..568ad560c112 100644
--- a/drivers/nvme/target/nvmet.h
+++ b/drivers/nvme/target/nvmet.h
@@ -129,6 +129,7 @@ struct nvmet_ctrl {
 	struct kref		ref;
 	struct delayed_work	ka_work;
 	struct work_struct	fatal_err_work;
+	struct work_struct	free_work;
 
 	struct nvmet_fabrics_ops *ops;
 
-- 
2.13.1