netdev - Re: Kernel rwlock design, Multicore and IGMP

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Date:	Fri, 12 Nov 2010 08:27:54 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Américo Wang <xiyou.wangcong@...il.com>
Cc:	Cypher Wu <cypher.w@...il.com>, linux-kernel@...r.kernel.org,
	netdev <netdev@...r.kernel.org>
Subject: Re: Kernel rwlock design, Multicore and IGMP

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.

> 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.

In the IGMP case, it should be easy for the task owning a read_lock() to
pass a parameter to the called function saying 'I already own the
read_lock(), dont try to re-acquire it'

A RCU conversion is far more complex.



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