netdev - Re: netlink circular locking dependency

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Date:	Mon, 16 Jun 2008 23:48:01 +0200
From:	Patrick McHardy <kaber@...sh.net>
To:	Jarek Poplawski <jarkao2@...il.com>
CC:	Marcel Holtmann <marcel@...tmann.org>, netdev@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>, Thomas Graf <tgraf@...g.ch>
Subject: Re: netlink circular locking dependency

Jarek Poplawski wrote:
> Marcel Holtmann wrote, On 06/14/2008 02:35 PM:
> ...
>   
>> =======================================================
>> [ INFO: possible circular locking dependency detected ]
>> 2.6.26-rc2 #5
>> -------------------------------------------------------
>> hcid/4136 is trying to acquire lock:
>>  (genl_mutex){--..}, at: [<c0000000002ace4c>] .ctrl_dumpfamily+0x74/0x174
>>
>> but task is already holding lock:
>>  (nlk->cb_mutex){--..}, at: [<c0000000002a766c>] .netlink_dump+0x58/0x27c
>>
>> which lock already depends on the new lock.
>>     
> ...
>
> Hi,
>
> IMHO it looks like a real lockup threat. Probably it needs something
> better, but for now here is my simplistic patch proposal for testing.
>   
So we have:

genl_rcv()            : take genl_mutex
genl_rcv_msg()        : call netlink_dump_start() while holding genl_mutex
netlink_dump_start(),
netlink_dump()        : take nlk->cb_mutex
ctrl_dumpfamily()     : try to detect this case and not take genl_mutex a
                        second time

netlink_rcv()         : call netlink_dump
netlink_dump          : take nlk->cb_mutex
ctrl_dumpfamily()     : take genl_mutex

which is a real bug.

It seems the best fix is to use genl_mutex for the netlink cb_mutex,
drop genl_mutex before calling netlink_dump_start and don't take it
in ctrl_dumpfamily, relying completely on af_netlink.c for dump
locking. Unfortunately this creates a race since the ops passed to
netlink_dump_start are also protect by the mutex, so this patch
is just for testing whether it fixes the warning.

On second though - that race seems to be present already since
the ops can be unregistered and the module unloaded while a dump
is in progress.


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