Commit 850a545b authored Oct 13, 2011 by Eric W. Biederman Committed by David S. Miller Oct 19, 2011

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 <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent 06a59ecb

net/core/dev.c

+7 −1

Original line number	Diff line number	Diff line
		@@ -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);