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@xxxxxxxxxxxxx -- Linux-cachefs mailing list Linux-cachefs@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/linux-cachefs
