netdev - Re: [PATCH] net: Move rcu_barrier from rollback_registered_many to netdev_run

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Date:	Wed, 19 Oct 2011 16:59:54 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	ebiederm@...ssion.com
Cc:	eric.dumazet@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH] net: Move rcu_barrier from rollback_registered_many to
 netdev_run_todo.

From: ebiederm@...ssion.com (Eric W. Biederman)
Date: Fri, 14 Oct 2011 01:25:23 -0700

> 
> 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@...ssion.com>

Applied to net-next, thanks Eric.
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