netdev - Re: Kernel rwlock design, Multicore and IGMP

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Date:	Fri, 12 Nov 2010 15:13:23 +0800
From:	Américo Wang <xiyou.wangcong@...il.com>
To:	Cypher Wu <cypher.w@...il.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	linux-kernel@...r.kernel.org, netdev <netdev@...r.kernel.org>
Subject: Re: Kernel rwlock design, Multicore and IGMP

On Fri, Nov 12, 2010 at 11:32:59AM +0800, Cypher Wu wrote:
>On Thu, Nov 11, 2010 at 11:23 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> Le jeudi 11 novembre 2010 à 21:49 +0800, Cypher Wu a écrit :
>>
>> Hi
>>
>> CC netdev, since you ask questions about network stuff _and_ rwlock
>>
>>
>>> I'm using TILEPro and its rwlock in kernel is a liitle different than
>>> other platforms. It have a priority for write lock that when tried it
>>> will block the following read lock even if read lock is hold by
>>> others. Its code can be read in Linux Kernel 2.6.36 in
>>> arch/tile/lib/spinlock_32.c.
>>
>> This seems a bug to me.
>>
>> read_lock() can be nested. We used such a schem in the past in iptables
>> (it can re-enter itself),
>> and we used instead a spinlock(), but with many discussions with lkml
>> and Linus himself if I remember well.
>>
>It seems not a problem that read_lock() can be nested or not since
>rwlock doesn't have 'owner', it's just that should we give
>write_lock() a priority than read_lock() since if there have a lot
>read_lock()s then they'll starve write_lock().
>We should work out a well defined behavior so all the
>platform-dependent raw_rwlock has to design under that principle.

It is a known weakness of rwlock, it is designed like that. :)

The solution is to use RCU or seqlock, but I don't think seqlock
is proper for this case you described. So, try RCU lock.
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