netdev - Re: [PATCH 1/1][PKT_CLS] Avoid multiple tree locks

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Date:	Wed, 21 Mar 2007 13:35:25 +0100
From:	Patrick McHardy <kaber@...sh.net>
To:	hadi@...erus.ca
CC:	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	Thomas Graf <tgraf@...g.ch>
Subject: Re: [PATCH 1/1][PKT_CLS] Avoid multiple tree locks

Patrick McHardy wrote:
>>Its harmless since its a read lock, which can be nested. I actually
>>don't see any need for qdisc_tree_lock at all, all changes and all
>>walking is done under the RTNL, which is why I've removed it in
>>my (upcoming) patches. I suggest to leave it as is for now so I
>>don't need to change the __qdisc_lookup back to qdisc_lookup in
>>2.6.22.
> 
> 
> 
> Alexey just explained to me why we do need qdisc_tree_lock in private
> mail. While dumping only the first skb is filled under the RTNL,
> while filling further skbs we don't hold the RTNL anymore. So I will
> probably have to drop that patch.

What we could do is replace the netlink cb_lock spinlock by a
user-supplied mutex (supplied to netlink_kernel_create, rtnl_mutex
in this case). That would put the entire dump under the rtnl and
allow us to get rid of qdisc_tree_lock and avoid the need to take
dev_base_lock during qdisc dumping. Same in other spots like
rtnl_dump_ifinfo, inet_dump_ifaddr, ...

What do you think?
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