Commit 8de1ee7e authored Nov 06, 2015 by Cody P Schafer Committed by Linus Torvalds Nov 06, 2015

rbtree: clarify documentation of rbtree_postorder_for_each_entry_safe()



I noticed that commit a20135ff ("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 <dev@codyps.com>
Cc: John de la Garza <john@jjdev.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 90224350

include/linux/rbtree.h

+10 −2

Original line number	Diff line number	Diff line
		@@ -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); \

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