netdev - Re: Re: Re: [RFC PATCH 1/1] IPVS netns shutdown/startup dead-lock

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Date:	Tue, 14 Jun 2011 11:47:16 +0200 (CEST)
From:	"Hans Schillstrom" <hans@...illstrom.com>
To:	"Julian Anastasov" <ja@....bg>
Cc:	"Hans Schillstrom" <hans.schillstrom@...csson.com>,
	horms@...ge.net.au, wensong@...ux-vs.org,
	lvs-devel@...r.kernel.org, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org
Subject: Re: Re: Re: [RFC PATCH 1/1] IPVS netns shutdown/startup 
	dead-lock


Hello,
>
>On Tue, 14 Jun 2011, Hans Schillstrom wrote:
>
>> >On Mon, 13 Jun 2011, Hans Schillstrom wrote:
>> >
>> >> ip_vs_mutext is used by both netns shutdown code and startup
>> >> and both implicit uses sk_lock-AF_INET mutex.
>> >> 
>> >> cleanup CPU-1         startup CPU-2
>> >> ip_vs_dst_event()     ip_vs_genl_set_cmd()
>> >>  sk_lock-AF_INET     __ip_vs_mutex
>> >>                      sk_lock-AF_INET
>> >> __ip_vs_mutex
>> >> * DEAD LOCK *
>> >
>> >	So, sk_lock-AF_INET is locked before calling
>> >ip_vs_dst_event ? Do you have a backtrace for this case?
>> 
>> Yes plenty this one is with lockdep
>> 
>> Chain exists of:
>>   rtnl_mutex --> __ip_vs_mutex --> sk_lock-AF_INET
>> 
>>  Possible unsafe locking scenario:
>> 
>>        CPU0                    CPU1
>>        ----                    ----
>>   lock(sk_lock-AF_INET);
>>                                lock(__ip_vs_mutex);
>>                                lock(sk_lock-AF_INET);
>>   lock(rtnl_mutex);
>> 
>>  *** DEADLOCK ***
>> 
>> 3 locks held by ipvsadm/993:
>>  #0:  (genl_mutex){+.+.+.}, at: [<ffffffff812edc52>] genl_lock+0x17/0x19
>>  #1:  (__ip_vs_mutex){+.+.+.}, at: [<ffffffff81307dcb>] ip_vs_genl_set_cmd+0xe1/0x3a3
>>  #2:  (sk_lock-AF_INET){+.+.+.}, at: [<ffffffff8130ffc1>] start_sync_thread+0x3ec/0x5ff
>
>	I see
>
>> >> ip_vs_mutex per name-space seems to be a more future proof solution.
>> >
>> >	Global mutex protects some global lists such as
>> >virtual services. If your patch works, better way to fix this problem
>> >is to use some new mutex. May be we can move the IPVS_CMD_NEW_DAEMON,
>> >IPVS_CMD_DEL_DAEMON and IP_VS_SO_GET_DAEMON code before the
>> >__ip_vs_mutex locking. This mutex should be used for start_sync_thread,
>> >stop_sync_thread, ip_vs_genl_dump_daemons and IP_VS_SO_GET_DAEMON.
>> >For example, ip_vs_sync_mutex.
>> 
>> I think we should avoid global mutexes as a rule of tumb, 
>> because it's realy hard to keep track of all possible cases 
>> that can occur when multiple netns is alive and/or goes up and down.
>> 
>> There might be more suprises while a netns exits (in terms of locks)...
>> my gut feeling is, avoid global locks as long as possible.
>
>	There should not be a problem between two netns when
>using global mutexes. 

as long as the locking occurs in the same order :-)

>And there are no many places in IPVS
>where other modules are accessed.
>
>> >	Note that __ip_vs_sync_cleanup is missing a
>> >__ip_vs_mutex lock. We have to use the new mutex there.
>> 
>> OK
>> 
>> >
>> >> Which one should be used ?
>> >
>> >	For now __ip_vs_mutex should be global ...
>> 
>> I do agree, but in the long term I vote for mutex per netns.
>
>	It will not help because the problem does not happen
>between two netspaces but between ipvs and other modules.
>The same problem would happen even if __ip_vs_mutex was
>pernet mutex. 

Actually it's between userspaces that uses different netns
i.e. when starting a thread and exit a container (with different namespaces)
This bug would not have occured if a per netns mutex had been used.

>So, lets try with new mutex.

OK, I missed the reading of thread status, i.e. a sync_mutex is needed

There is no need for a lock(mutex) in ip_vs_sync_net_cleanup() before stoping threads,
because when it's called no user processes exits in that namespace.

Regards
Hans Schillstrom





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