[PATCH net-next v2] rtnetlink: comment the RCU waits in unregister paths

[Andrea Parri <parri.andrea@gmail.com>]

rtnl_unregister_all() and rtnl_af_unregister() wait for RCU readers
before tearing down what those readers use, but do not say which
readers they are waiting for.

A missing wait is then easy to overlook: rtnl_link_unregister() lacked
its synchronize_rcu() until commit 3b70edea393d ("rtnetlink: wait for
RCU readers in rtnl_link_unregister()"). Add a comment to both waits,
naming the readers each one waits for.

Suggested-by: Eric Dumazet <edumazet@kernel.org>
Link: https://lore.kernel.org/CAL4WiioFMQN4troVjQjCmE6Z9kwwb95SnoEupjD=PPSK6Q5f6w@mail.gmail.com
Assisted-by: LLM
Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
---
v2:
- Drop the comments on synchronize_srcu(), its argument already names
  the readers (Kuniyuki)
- Name rtnetlink_rcv_msg() and try_module_get() in the
  rtnl_unregister_all() comment (Kuniyuki)
- Mention the rtnl_af_ops walkers that do not take ops->srcu in the
  rtnl_af_unregister() comment

v1: https://lore.kernel.org/20261008124423.11543-1-parri.andrea@gmail.com

 net/core/rtnetlink.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index 15bffafbf0a9c..d73dbc2c8ba0d 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -526,6 +526,9 @@ void rtnl_unregister_all(int protocol)
 	}
 	rtnl_unlock();
 
+	/* Wait for rtnl_get_link() calls that rtnetlink_rcv_msg() makes
+	 * under RCU, before try_module_get().
+	 */
 	synchronize_net();
 
 	kfree(tab);
@@ -843,6 +846,9 @@ void rtnl_af_unregister(struct rtnl_af_ops *ops)
 	list_del_rcu(&ops->list);
 	spin_unlock(&rtnl_af_ops_lock);
 
+	/* Wait for rtnl_af_ops walkers that invoke callbacks without
+	 * ops->srcu, and for rtnl_af_lookup() readers not yet holding it.
+	 */
 	synchronize_rcu();
 	synchronize_srcu(&ops->srcu);
 	cleanup_srcu_struct(&ops->srcu);
-- 
2.53.0