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    SubjectRe: 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....


    Dave Chinner

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