netdev - Re: IGMP and rwlock: Dead ocurred again on TILEPro

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Message-Id: <20110216.214625.189707123.davem@davemloft.net>
Date:	Wed, 16 Feb 2011 21:46:25 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	xiyou.wangcong@...il.com
Cc:	cypher.w@...il.com, linux-kernel@...r.kernel.org,
	cmetcalf@...era.com, eric.dumazet@...il.com, netdev@...r.kernel.org
Subject: Re: IGMP and rwlock: Dead ocurred again on TILEPro

From: Américo Wang <xiyou.wangcong@...il.com>
Date: Thu, 17 Feb 2011 13:42:37 +0800

> On Thu, Feb 17, 2011 at 01:04:14PM +0800, Cypher Wu wrote:
>>>
>>> Have you turned CONFIG_LOCKDEP on?
>>>
>>> I think Eric already converted that rwlock into RCU lock, thus
>>> this problem should disappear. Could you try a new kernel?
>>>
>>> Thanks.
>>>
>>
>>I haven't turned CONFIG_LOCKDEP on for test since I didn't get too
>>much information when we tried to figured out the former deadlock.
>>
>>IGMP used read_lock() instead of read_lock_bh() since usually
>>read_lock() can be called recursively, and today I've read the
>>implementation of MIPS, it's should also works fine in that situation.
>>The implementation of TILEPro cause problem since after it use TNS set
>>the lock-val to 1 and hold the original value and before it re-set
>>lock-val a new value, it a race condition window.
>>
> 
> I see no reason why you can't call read_lock_bh() recursively,
> read_lock_bh() is roughly equalent to local_bh_disable() + read_lock(),
> both can be recursive.
> 
> But I may miss something here. :-/

IGMP is doing this so that taking the read lock does not stop packet
processing.

TILEPro's rwlock implementation is simply buggy and needs to be fixed.
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