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

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Message-ID: <AANLkTind92uwcigzmDn8yn9a22exDy7zcreGQ5-6NLV-@mail.gmail.com>
Date:	Mon, 15 Nov 2010 15:22:02 +0800
From:	Cypher Wu <cypher.w@...il.com>
To:	Chris Metcalf <cmetcalf@...era.com>
Cc:	Américo Wang <xiyou.wangcong@...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

On Sun, Nov 14, 2010 at 7:03 AM, Chris Metcalf <cmetcalf@...era.com> wrote:
> On 11/12/2010 2:13 AM, Américo Wang wrote:
>> 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 :
>>>>> 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.
>>>> [...]
>>>>
>>> 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. :)
>
> Exactly.  The tile rwlock correctly allows recursively reacquiring the read
> lock.  But it does give priority to writers, for the (unfortunately
> incorrect) reasons Cypher Wu outlined above, e.g.:
>
> - Core A takes a read lock
> - Core B tries for a write lock and blocks new read locks
> - Core A tries for a (recursive) read lock and blocks
>
> Core A and B are now deadlocked.
>
> The solution is actually to simplify the tile rwlock implementation so that
> both readers and writers contend fairly for the lock.
>
> I'll post a patch in the next day or two for tile.
>
> --
> Chris Metcalf, Tilera Corp.
> http://www.tilera.com
>

We're looking forward to you patch. BTW: could your fix it up in Linux
2.6.26.7 which is not release in the normal kernel?

-- 
Cyberman Wu
http://www.meganovo.com
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