netdev - [PATCH v3 net-next 4/5] net: reduce RTNL hold duration in unregister_netdevice_many

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Message-ID: <20250114205531.967841-5-edumazet@google.com>
Date: Tue, 14 Jan 2025 20:55:30 +0000
From: Eric Dumazet <edumazet@...gle.com>
To: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>
Cc: netdev@...r.kernel.org, Simon Horman <horms@...nel.org>, eric.dumazet@...il.com, 
	Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH v3 net-next 4/5] net: reduce RTNL hold duration in
 unregister_netdevice_many_notify() (part 1)

Two synchronize_net() calls are currently done while holding RTNL.

This is source of RTNL contention in workloads adding and deleting
many network namespaces per second, because synchronize_rcu()
and synchronize_rcu_expedited() can use 60+ ms in some cases.

For cleanup_net() use, temporarily release RTNL
while calling the last synchronize_net().

This should be safe, because devices are no longer visible
to other threads at this point.

In any case, the new netdev_lock() / netdev_unlock()
infrastructure that we are adding should allow
to fix potential issues, with a combination
of a per-device mutex and dev->reg_state awareness.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 net/core/dev.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/core/dev.c b/net/core/dev.c
index f4dd92bed2223269053b6576e4954fcce218a2e5..574bd40f3a2bfcc6e43300fad669b1579d48039a 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -11602,6 +11602,7 @@ void unregister_netdevice_many_notify(struct list_head *head,
 	rtnl_drop_if_cleanup_net();
 	flush_all_backlogs();
 	rtnl_acquire_if_cleanup_net();
+	/* TODO: move this before the prior rtnl_acquire_if_cleanup_net() */
 	synchronize_net();

 	list_for_each_entry(dev, head, unreg_list) {
@@ -11662,7 +11663,9 @@ void unregister_netdevice_many_notify(struct list_head *head,
 #endif
 	}

+	rtnl_drop_if_cleanup_net();
 	synchronize_net();
+	rtnl_acquire_if_cleanup_net();

 	list_for_each_entry(dev, head, unreg_list) {
 		netdev_put(dev, &dev->dev_registered_tracker);
-- 
2.48.0.rc2.279.g1de40edade-goog