From 8de1ee7ebfb4979c6444e81273e12e7a972c367d Mon Sep 17 00:00:00 2001 From: Cody P Schafer Date: Fri, 6 Nov 2015 16:31:28 -0800 Subject: [PATCH] rbtree: clarify documentation of rbtree_postorder_for_each_entry_safe() I noticed that commit a20135ffbc44 ("writeback: don't drain bdi_writeback_congested on bdi destruction") added a usage of rbtree_postorder_for_each_entry_safe() in mm/backing-dev.c which appears to try to rb_erase() elements from an rbtree while iterating over it using rbtree_postorder_for_each_entry_safe(). Doing this will cause random nodes to be missed by the iteration because rb_erase() may rebalance the tree, changing the ordering that we're trying to iterate over. The previous documentation for rbtree_postorder_for_each_entry_safe() wasn't clear that this wasn't allowed, it was taken from the docs for list_for_each_entry_safe(), where erasing isn't a problem due to list_del() not reordering. Explicitly warn developers about this potential pit-fall. Note that I haven't fixed the actual issue that (it appears) the commit referenced above introduced (not familiar enough with that code). In general (and in this case), the patterns to follow are: - switch to rb_first() + rb_erase(), don't use rbtree_postorder_for_each_entry_safe(). - keep the postorder iteration and don't rb_erase() at all. Instead just clear the fields of rb_node & cgwb_congested_tree as required by other users of those structures. [akpm@linux-foundation.org: tweak comments] Signed-off-by: Cody P Schafer Cc: John de la Garza Cc: Michel Lespinasse Cc: Peter Zijlstra Cc: Rusty Russell Cc: Tejun Heo Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- include/linux/rbtree.h | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-) diff --git a/include/linux/rbtree.h b/include/linux/rbtree.h index 830c4992088d..a5aa7ae671f4 100644 --- a/include/linux/rbtree.h +++ b/include/linux/rbtree.h @@ -101,13 +101,21 @@ static inline void rb_link_node_rcu(struct rb_node *node, struct rb_node *parent }) /** - * rbtree_postorder_for_each_entry_safe - iterate over rb_root in post order of - * given type safe against removal of rb_node entry + * rbtree_postorder_for_each_entry_safe - iterate in post-order over rb_root of + * given type allowing the backing memory of @pos to be invalidated * * @pos: the 'type *' to use as a loop cursor. * @n: another 'type *' to use as temporary storage * @root: 'rb_root *' of the rbtree. * @field: the name of the rb_node field within 'type'. + * + * rbtree_postorder_for_each_entry_safe() provides a similar guarantee as + * list_for_each_entry_safe() and allows the iteration to continue independent + * of changes to @pos by the body of the loop. + * + * Note, however, that it cannot handle other modifications that re-order the + * rbtree it is iterating over. This includes calling rb_erase() on @pos, as + * rb_erase() may rebalance the tree, causing us to miss some nodes. */ #define rbtree_postorder_for_each_entry_safe(pos, n, root, field) \ for (pos = rb_entry_safe(rb_first_postorder(root), typeof(*pos), field); \ -- 2.20.1