From mboxrd@z Thu Jan  1 00:00:00 1970
From: Trond Myklebust <Trond.Myklebust@netapp.com>
Subject: Re: [PATCH 5/6] IMA: use rbtree instead of radix tree for inode
 information cache
Date: Wed, 20 Oct 2010 18:47:55 -0400
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Cc: Dave Chinner <david@fromorbit.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Eric Paris <eparis@redhat.com>, linux-kernel@vger.kernel.org,
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	hpa@zytor.com, akpm@linux-foundation.org, mingo@elte.hu,
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To: Linus Torvalds <torvalds@linux-foundation.org>
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On Wed, 2010-10-20 at 15:22 -0700, Linus Torvalds wrote:
> On Wed, Oct 20, 2010 at 3:05 PM, Dave Chinner <david@fromorbit.com> wrote:
> >
> > /me wanders off to look at converting the xfs buffer cache rbtrees
> > to RCU....
> 
> Look out for livelocks, though. And yes, they can happen.
> 
> So rather than a loop, one option is to do basically
> 
>   rcu_read_lock();
>   seq = read_seqbegin();
> 
>   .. do lookup ..
> 
>   need_lock = read_seqretry(seq);
>   rcu_read_unlock();
> 
>   if (need_lock) {
>     get_real_lock();
> 
>     .. do lookup ..
> 
>     drop_real_lock();
>  }
> 
> which just falls back to a locked access if the rcu model doesn't work.
> 
>                              Linus

That is a really interesting alternative to traditional locking. Could
we perhaps document it in Documentation/rbtree.txt?

Cheers
   Trond