nvme: Avoid flush dependency in delete controller flow

The nvme_delete_ctrl() function queues a work item on a MEM_RECLAIM
queue (nvme_delete_wq), which eventually calls cleanup_srcu_struct(),
which in turn flushes a delayed work from an !MEM_RECLAIM queue. This
is unsafe as we might trigger deadlocks under severe memory pressure.

Since we don't ever invoke call_srcu(), it is safe to use the shiny new
_quiesced() version of srcu cleanup, thus avoiding that flush dependency.
This commit makes that change.

Signed-off-by: Nitzan Carmi <nitzanc@mellanox.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>

drivers/nvme/host/core.c

@@ -349,7 +349,7 @@ static void nvme_free_ns_head(struct kref *ref)
 	nvme_mpath_remove_disk(head);
 	ida_simple_remove(&head->subsys->ns_ida, head->instance);
 	list_del_init(&head->entry);
-	cleanup_srcu_struct(&head->srcu);
+	cleanup_srcu_struct_quiesced(&head->srcu);
 	kfree(head);
 }