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

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Date:	Wed, 20 Oct 2010 15:22:21 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Dave Chinner <david@...morbit.com>
Cc:	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, 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
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