linux-kernel - Re: Kernel rwlock design, Multicore and IGMP

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Date:	Fri, 12 Nov 2010 19:06:47 +0800
From:	Cypher Wu <cypher.w@...il.com>
To:	Américo Wang <xiyou.wangcong@...il.com>
Cc:	Yong Zhang <yong.zhang0@...il.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	linux-kernel@...r.kernel.org, netdev <netdev@...r.kernel.org>
Subject: Re: Kernel rwlock design, Multicore and IGMP

2010/11/12 Américo Wang <xiyou.wangcong@...il.com>:
> On Fri, Nov 12, 2010 at 05:09:45PM +0800, Yong Zhang wrote:
>>On Fri, Nov 12, 2010 at 4:19 PM, Américo Wang <xiyou.wangcong@...il.com> wrote:
>>> On Fri, Nov 12, 2010 at 08:27:54AM +0100, Eric Dumazet wrote:
>>>>Le vendredi 12 novembre 2010 à 15:13 +0800, Américo Wang a écrit :
>>>>> 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.
>>>>>
>>>>
>>>>AFAIK, Lockdep allows read_lock() to be nested.
>>>>
>>>>> It is a known weakness of rwlock, it is designed like that. :)
>>>>>
>>>>
>>>>Agreed.
>>>>
>>>
>>> Just for record, both Tile and X86 implement rwlock with a write-bias,
>>> this somewhat reduces the write-starvation problem.
>>
>>Are you sure(on x86)?
>>
>>It seems that we never realize writer-bias rwlock.
>>
>
> Try
>
> % grep RW_LOCK_BIAS -nr arch/x86
>
> *And* read the code to see how it works. :)
>
> Note, on Tile, it uses a little different algorithm.
>

It seems that rwlock on x86 and tile have different behavior, x86 use
RW_LOCK_BIAS, when read_lock() it will test if the lock is 0, and if
so then the read_lock() have to 'spinning', otherwise it dec the lock;
when write_lock() tried it first check if lock is It seems that rwlock
on x86 and tile have different behavior, x86 use RW_LOCK_BIAS and if
so, set lock to 0 and continue, otherwise it will 'spinning'.
I'm not very familiar with x86 architecture, but the code seems like
working that way.
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