[PATCH v2 2/2] nvme: wait for SRCU readers before splicing the namespace list

[Qiliang Yuan <odys.yuan@gmail.com>]

Readers walk ctrl->namespaces under ctrl->srcu, and some of them sleep
there, e.g. nvme_wait_freeze() waiting for I/O to drain.

nvme_remove_namespaces() moves the list onto its on-stack ns_list with
list_splice_init_rcu(), which only waits for RCU readers before
relinking the entries. A reader still on the last namespace then follows
its next pointer to ns_list instead of ctrl->namespaces and treats the
stack as a namespace. Deleting a controller while a Format NVM
passthrough waits for a delayed read on the last namespace gives:

  BUG: KASAN: stack-out-of-bounds in nvme_wait_freeze+0x102/0x120
  Read of size 8 at addr ff110001d2f3fc38 by task nvme/4413
  Call Trace:
   nvme_wait_freeze
   nvme_passthru_start
   nvme_submit_user_cmd
   nvme_user_cmd
   nvme_dev_ioctl
  BUG: unable to handle page fault for address: ffffffff8af04850

Use list_splice_init_srcu() instead, which waits for the SRCU readers.
This also takes the RCU grace period out of every controller deletion.
Deleting 64 tcp controllers with one namespace each, median of 3 runs on
each of two boots per kernel:

                                      before        after
  nvme disconnect-all (ms)            3491 / 3509   2996 / 3165
  nvme_remove_namespaces() avg (ms)   39.0 / 40.5   31.9 / 34.7

Fixes: be647e2c76b2 ("nvme: use srcu for iterating namespace list")
Cc: stable@vger.kernel.org
Signed-off-by: Qiliang Yuan <odys.yuan@gmail.com>
---
 drivers/nvme/host/core.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index a7a4d09644c13..cdb576db23809 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -4755,9 +4755,8 @@ void nvme_remove_namespaces(struct nvme_ctrl *ctrl)
 	nvme_change_ctrl_state(ctrl, NVME_CTRL_DELETING_NOIO);
 
 	mutex_lock(&ctrl->namespaces_lock);
-	list_splice_init_rcu(&ctrl->namespaces, &ns_list, synchronize_rcu);
+	list_splice_init_srcu(&ctrl->namespaces, &ns_list, &ctrl->srcu);
 	mutex_unlock(&ctrl->namespaces_lock);
-	synchronize_srcu(&ctrl->srcu);
 
 	list_for_each_entry_safe(ns, next, &ns_list, list)
 		nvme_ns_remove(ns);

-- 
2.43.0