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

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Date:	Wed, 21 Mar 2007 15:04:35 +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:
>>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, ...

These (compile tested) patches demonstrate the idea. The first one
lets netlink_kernel_create users specify a mutex that should be
held during dump callbacks, the second one uses this for rtnetlink
and changes inet_dump_ifaddr for demonstration.

A complete patch would allow us to simplify locking in lots of
spots, all rtnetlink users currently need to implement extra
locking just for the dump functions, and a number of them
already get it wrong and seem to rely on the rtnl.

If there are no objections to this change I'm going to update
the second patch to include all rtnetlink users.

View attachment "01.diff" of type "text/x-diff" (1515 bytes)

View attachment "02.diff" of type "text/x-diff" (1138 bytes)