From: Dave Chinner <david@fromorbit.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Eric Paris <eparis@redhat.com>,
linux-kernel@vger.kernel.org,
linux-security-module@vger.kernel.org,
linux-fsdevel@vger.kernel.org, hch@infradead.org,
zohar@us.ibm.com, warthog9@kernel.org, jmorris@namei.org,
kyle@mcmartin.ca, hpa@zytor.com, akpm@linux-foundation.org,
torvalds@linux-foundation.org, mingo@elte.hu,
viro@zeniv.linux.org.uk
Subject: Re: [PATCH 5/6] IMA: use rbtree instead of radix tree for inode information cache
Date: Thu, 21 Oct 2010 09:05:57 +1100 [thread overview]
Message-ID: <20101020220557.GN32255@dastard> (raw)
In-Reply-To: <1287574269.3488.11.camel@twins>
On Wed, Oct 20, 2010 at 01:31:09PM +0200, Peter Zijlstra wrote:
> On Wed, 2010-10-20 at 10:17 +1100, Dave Chinner wrote:
> > On Tue, Oct 19, 2010 at 06:58:39PM -0400, Eric Paris wrote:
> > > @@ -36,12 +63,11 @@ struct ima_iint_cache *ima_iint_find_get(struct inode *inode)
> > > struct ima_iint_cache *iint;
> > >
> > > rcu_read_lock();
> > > - iint = radix_tree_lookup(&ima_iint_store, (unsigned long)inode);
> > > - if (!iint)
> > > - goto out;
> > > - kref_get(&iint->refcount);
> > > -out:
> > > + iint = __ima_iint_find(inode);
> > > + if (iint)
> > > + kref_get(&iint->refcount);
> > > rcu_read_unlock();
> > > +
> >
> > This is wrong - the rbtree is protected only by the ima_iint_lock(),
> > not RCU. Hence you can't do lockless lookups on an rbtree in this
> > manner as they will race with inserts and deletes.
>
> Correct, what can be made to work is combine RCU with a seqlock. Retry
> the lookup using read_seqretry(), RCU here helps to ensure you're not
> stepping on already freed memory.
>
>
> So, tree modification does:
>
> write_seqlock();
> /* frob RB-tree, using call_rcu() for frees where needed */
> write_sequnlock();
>
> Lookup does:
>
> unsigned seq;
>
> rcu_read_lock()
> again;
> seq = read_seqbegin();
>
> /* RB-tree lookup */
>
> if (read_seqretry(seq))
> goto again;
>
> rcu_read_unlock();
>
> return obj;
Nice trick, Peter. Thanks for sharing that - I'll definitely find
that useful. :)
/me wanders off to look at converting the xfs buffer cache rbtrees
to RCU....
Cheers,
Dave.
--
Dave Chinner
david@fromorbit.com
next prev parent reply other threads:[~2010-10-20 22:09 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-10-19 22:58 [PATCH 1/6] IMA: move read/write counters into struct inode Eric Paris
2010-10-19 22:58 ` [PATCH 2/6] IMA: drop the inode opencount since it isn't needed for operation Eric Paris
2010-10-19 22:58 ` [PATCH 3/6] IMA: use unsigned int instead of long for counters Eric Paris
2010-10-19 22:58 ` [PATCH 4/6] IMA: only allocate iint when needed Eric Paris
2010-10-20 3:53 ` Al Viro
2010-10-19 22:58 ` [PATCH 5/6] IMA: use rbtree instead of radix tree for inode information cache Eric Paris
2010-10-19 23:17 ` Dave Chinner
2010-10-20 11:31 ` Peter Zijlstra
2010-10-20 22:05 ` Dave Chinner [this message]
2010-10-20 22:22 ` Linus Torvalds
2010-10-20 22:47 ` Trond Myklebust
2010-10-21 0:58 ` Linus Torvalds
2010-10-21 2:17 ` Dave Chinner
2010-10-19 22:58 ` [PATCH 6/6] IMA: use i_writecount rather than a private counter Eric Paris
2010-10-20 0:25 ` [PATCH 1/6] IMA: move read/write counters into struct inode Linus Torvalds
2010-10-23 3:01 ` Eric Paris
2010-10-24 6:52 ` Mimi Zohar
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