Re: IGMP and rwlock: Dead ocurred again on TILEPro

[David Miller <davem@davemloft.net>]

From: Américo Wang <xiyou.wangcong@gmail.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.