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

[Andrea Parri]

rtnl_link_unregister(), rtnl_af_unregister() and rtnl_unregister_all()
wait for readers before tearing down what those readers use, but most
of these waits 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 each wait
that lacks one, naming the readers it waits for.

Suggested-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Link: https://lore.kernel.org/CAL4WiioFMQN4troVjQjCmE6Z9kwwb95SnoEupjD=PPSK6Q5f6w@xxxxxxxxxxxxxx
Assisted-by: LLM
Signed-off-by: Andrea Parri <parri.andrea@xxxxxxxxx>
---
 net/core/rtnetlink.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index 15bffafbf0a9c..80f8577816760 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() callers under RCU that may still
+	 * dereference tab.
+	 */
 	synchronize_net();
 
 	kfree(tab);
@@ -711,6 +714,7 @@ void rtnl_link_unregister(struct rtnl_link_ops *ops)
 	 * not yet taken ops->srcu.
 	 */
 	synchronize_rcu();
+	/* Wait for rtnl_link_ops_get() callers to call rtnl_link_ops_put(). */
 	synchronize_srcu(&ops->srcu);
 	cleanup_srcu_struct(&ops->srcu);
 
@@ -843,7 +847,11 @@ void rtnl_af_unregister(struct rtnl_af_ops *ops)
 	list_del_rcu(&ops->list);
 	spin_unlock(&rtnl_af_ops_lock);
 
+	/* Wait for RCU walkers of rtnl_af_ops, including rtnl_af_lookup()
+	 * readers that found ops but have not yet taken ops->srcu.
+	 */
 	synchronize_rcu();
+	/* Wait for rtnl_af_lookup() callers to call rtnl_af_put(). */
 	synchronize_srcu(&ops->srcu);
 	cleanup_srcu_struct(&ops->srcu);
 }
-- 
2.53.0