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

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Message-ID: <AANLkTi=Z6=5v3vKwu1HZsB1CBQ_acdmjbVG4PZbfM_OC@mail.gmail.com>
Date:	Thu, 17 Feb 2011 14:42:35 +0800
From:	Cypher Wu <cypher.w@...il.com>
To:	Américo Wang <xiyou.wangcong@...il.com>
Cc:	linux-kernel@...r.kernel.org, Chris Metcalf <cmetcalf@...era.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	netdev <netdev@...r.kernel.org>
Subject: Re: Fwd: IGMP and rwlock: Dead ocurred again on TILEPro

On Thu, Feb 17, 2011 at 1:42 PM, Américo Wang <xiyou.wangcong@...il.com> wrote:
> 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. :-/
>

Of course read_lock_bh() can be called recursively, but read_lock() is
already enough for IGMP, the only reason for that deadlock is because
using TNS instruction set the value to 1 cause another race condition.

-- 
Cyberman Wu
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