linux-kernel - Re: [PATCH 5/6] IMA: use rbtree instead of radix tree for inode information cache

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Message-ID: <1287614875.23812.7.camel@heimdal.trondhjem.org>
Date:	Wed, 20 Oct 2010 18:47:55 -0400
From:	Trond Myklebust <Trond.Myklebust@...app.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Dave Chinner <david@...morbit.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Eric Paris <eparis@...hat.com>, linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, hch@...radead.org, zohar@...ibm.com,
	warthog9@...nel.org, jmorris@...ei.org, kyle@...artin.ca,
	hpa@...or.com, akpm@...ux-foundation.org, mingo@...e.hu,
	viro@...iv.linux.org.uk
Subject: Re: [PATCH 5/6] IMA: use rbtree instead of radix tree for inode
 information cache

On Wed, 2010-10-20 at 15:22 -0700, Linus Torvalds wrote:
> On Wed, Oct 20, 2010 at 3:05 PM, Dave Chinner <david@...morbit.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

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