netdev - Re: [PATCH 1/2] Remove unnecessary locks from rtnetlink

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Message-Id: <20080205.175337.266368079.davem@davemloft.net>
Date:	Tue, 05 Feb 2008 17:53:37 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	panther@...abit.hu
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH 1/2] Remove unnecessary locks from rtnetlink

From: Laszlo Attila Toth <panther@...abit.hu>
Date: Fri,  1 Feb 2008 17:07:33 +0100

> The do_setlink() function is protected by rtnl, additional locks are unnecessary.
> and the set_operstate() function is called from protected parts. Locks removed
> from both functions.
> 
> The set_operstate() is also called from rtnl_create_link() and from no other places.
> In rtnl_create_link() none of the changes is protected by set_lock_bh() except
> inside set_operstate(), different locking scheme is not necessary
> for the operstate.
> 
> Signed-off-by: Laszlo Attila Toth <panther@...abit.hu>

The protection using dev_base_lock() is needed.

When analyzing cases like this you need to also look at other code
paths outside of rtnetlink that access ->operstate and ->link_mode,
you obviously didn't do this.

For example, net/core/net-sysfs.c takes a read lock on dev_base_lock
in order to fetch a stable copy of both netif_running() and
dev->operstate at the same time.

Similar write locking to protect dev->operstate is made by
net/core/link_watch.c:rfc2863_policy(), for the same reason rtnetlink
has to make this locking.

You therefore cannot remove it.

This invalidates your second patch so I'm dropping that as well.
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