[PATCH net-next V6 03/14] devlink: Reverse locking order for nested instances

[Tariq Toukan <tariqt@nvidia.com>]

From: Cosmin Ratiu <cratiu@nvidia.com>

Commit [1] defined the locking expectations for nested devlink
instances: the nested-in devlink instance lock needs to be acquired
before the nested devlink instance lock. The code handling devlink rels
was architected with that assumption in mind.

There are no actual users of double locking yet but that is about to
change in the upcoming patches in the series.

Code operating on nested devlink instances will require also obtaining
the nested-in instance lock, but such code may already be called from a
variety of places with the nested devlink instance lock. Then, there's
no way to acquire the nested-in lock other than making sure that all
callers acquire it first.

Reversing the nested lock order allows incrementally acquiring the
nested-in instance lock when needed (perhaps even a chain of locks up to
the root) without affecting any caller.

The only affected use of nesting is devlink_nl_nested_fill(), which
iterates over nested devlink instances with the RCU lock, without
locking them, so there's no possibility of deadlock.

So this commit just updates a comment regarding the nested locks.

[1] commit c137743bce02b ("devlink: introduce object and nested devlink
relationship infra")

Signed-off-by: Cosmin Ratiu <cratiu@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
---
 net/devlink/core.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/net/devlink/core.c b/net/devlink/core.c
index 58093f49c090..6ae62c7f2a80 100644
--- a/net/devlink/core.c
+++ b/net/devlink/core.c
@@ -178,9 +178,7 @@ int devlink_rel_nested_in_add(u32 *rel_index, u32 devlink_index,
  * a notification of a change of this object should be sent
  * over netlink. The parent devlink instance lock needs to be
  * taken during the notification preparation.
- * However, since the devlink lock of nested instance is held here,
- * we would end with wrong devlink instance lock ordering and
- * deadlock. Therefore the work is utilized to avoid that.
+ * Since the parent may or may not be locked, 'work' is utilized.
  */
 void devlink_rel_nested_in_notify(struct devlink *devlink)
 {
-- 
2.40.1