From 850a545bd8a41648445bfc5541afe36ca105b0b8 Mon Sep 17 00:00:00 2001 From: "Eric W. Biederman" Date: Thu, 13 Oct 2011 22:25:23 +0000 Subject: [PATCH] net: Move rcu_barrier from rollback_registered_many to netdev_run_todo. This patch moves the rcu_barrier from rollback_registered_many (inside the rtnl_lock) into netdev_run_todo (just outside the rtnl_lock). This allows us to gain the full benefit of sychronize_net calling synchronize_rcu_expedited when the rtnl_lock is held. The rcu_barrier in rollback_registered_many was originally a synchronize_net but was promoted to be a rcu_barrier() when it was found that people were unnecessarily hitting the 250ms wait in netdev_wait_allrefs(). Changing the rcu_barrier back to a synchronize_net is therefore safe. Since we only care about waiting for the rcu callbacks before we get to netdev_wait_allrefs() it is also safe to move the wait into netdev_run_todo. This was tested by creating and destroying 1000 tap devices and observing /proc/lock_stat. /proc/lock_stat reports this change reduces the hold times of the rtnl_lock by a factor of 10. There was no observable difference in the amount of time it takes to destroy a network device. Signed-off-by: Eric W. Biederman Signed-off-by: David S. Miller --- net/core/dev.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/net/core/dev.c b/net/core/dev.c index cbb5918..09aef26 100644 --- a/net/core/dev.c +++ b/net/core/dev.c @@ -5235,7 +5235,7 @@ static void rollback_registered_many(struct list_head *head) dev = list_first_entry(head, struct net_device, unreg_list); call_netdevice_notifiers(NETDEV_UNREGISTER_BATCH, dev); - rcu_barrier(); + synchronize_net(); list_for_each_entry(dev, head, unreg_list) dev_put(dev); @@ -5748,6 +5748,12 @@ void netdev_run_todo(void) __rtnl_unlock(); + /* Wait for rcu callbacks to finish before attempting to drain + * the device list. This usually avoids a 250ms wait. + */ + if (!list_empty(&list)) + rcu_barrier(); + while (!list_empty(&list)) { struct net_device *dev = list_first_entry(&list, struct net_device, todo_list); -- 2.7.4