linux-kernel - Re: An incorrect assumption over radix_tree_tag

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Message-ID: <20100406233438.GH11036@dastard>
Date:	Wed, 7 Apr 2010 09:34:38 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	David Howells <dhowells@...hat.com>, paulmck@...ux.vnet.ibm.com,
	corbet@....net, linux-kernel@...r.kernel.org,
	linux-cachefs@...hat.com
Subject: Re: An incorrect assumption over radix_tree_tag_get()

On Wed, Apr 07, 2010 at 03:09:03AM +1000, Nick Piggin wrote:
> On Tue, Apr 06, 2010 at 05:19:49PM +0100, David Howells wrote:
> > 
> > Hi,
> > 
> > I think I've made a bad assumption over my usage of radix_tree_tag_get() in
> > fs/fscache/page.c.
> > 
> > I've assumed that radix_tree_tag_get() is protected from radix_tree_tag_set()
> > and radix_tree_tag_clear() by the RCU read lock.  However, now I'm not so
> > sure.  I think it's only protected against removal of part of the tree.
> > 
> > Can you confirm?
> 
> It is safe. Synchronization requirements for using the radix tree API
> are documented.

I don't think it is safe - I made modifications to XFS that modified
radix tree tags under a read lock (not RCU), but this resulted in
corrupted tag state as concurrent tag set/clear operations for
different slots propagated through the tree and got mixed up.
Christoph fixed the problem (f1f724e4b523d444c5a598d74505aefa3d6844d2)
by putting all tag modifications under the write lock.  I can't see
how doing tag modifications under RCU read locks is any safer than
doing it under a spinning read lock....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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