On Thu, May 08, 2014 at 04:53:30PM +0100, Paul E. McKenney wrote: > On Thu, May 08, 2014 at 04:29:48PM +0100, Catalin Marinas wrote: > > BTW, is it safe to have a union overlapping node->parent and > > node->rcu_head.next? I'm still staring at the radix-tree code but a > > scenario I have in mind is that call_rcu() has been raised for a few > > nodes, other CPU may have some reference to one of them and set > > node->parent to NULL (e.g. concurrent calls to radix_tree_shrink()), > > breaking the RCU linking. I can't confirm this theory yet ;) > > If this were reproducible, I would suggest retrying with non-overlapping > node->parent and node->rcu_head.next, but you knew that already. ;-) Reading the code, I'm less convinced about this scenario (though it's worth checking without the union). > But the usual practice would be to make node removal exclude shrinking. > And the radix-tree code seems to delegate locking to the caller. > > So, is the correct locking present in the page cache? The radix-tree > code seems to assume that all update operations for a given tree are > protected by a lock global to that tree. The calling code in mm/filemap.c holds mapping->tree_lock when deleting radix-tree nodes, so no concurrent calls. > Another diagnosis approach would be to build with > CONFIG_DEBUG_OBJECTS_RCU_HEAD=y, which would complain about double > call_rcu() invocations. Rumor has it that is is necessary to turn off > other kmem debugging for this to tell you anything -- I have seen cases > where the kmem debugging obscures the debug-objects diagnostics. Another test Jaegeuk could run (hopefully he has some time to look into this). Thanks for suggestions. -- Catalin -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>