writeback: remove broken rbtree_postorder_for_each_entry_safe() usage in cgwb_bdi_des...

a20135ffbc44 ("writeback: don't drain bdi_writeback_congested on bdi
destruction") added rbtree_postorder_for_each_entry_safe() which is
used to remove all entries; however, according to Cody, the iterator
isn't safe against operations which may rebalance the tree.  Fix it by
switching to repeatedly removing rb_first() until empty.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Cody P Schafer <dev@codyps.com>
Fixes: a20135ffbc44 ("writeback: don't drain bdi_writeback_congested on bdi destruction")
Link: http://lkml.kernel.org/g/1443997973-1700-1-git-send-email-dev@codyps.com
Signed-off-by: Jens Axboe <axboe@fb.com>

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 9e841399041a4b19f0d4b719b28856fc77ff777b..619984fc07ec32792349c7fe8aec7c8b56e3d2a3 100644 (file)
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -681,7 +681,7 @@ static int cgwb_bdi_init(struct backing_dev_info *bdi)
 static void cgwb_bdi_destroy(struct backing_dev_info *bdi)
 {
        struct radix_tree_iter iter;
-       struct bdi_writeback_congested *congested, *congested_n;
+       struct rb_node *rbn;
        void **slot;
 
        WARN_ON(test_bit(WB_registered, &bdi->wb.state));
@@ -691,9 +691,11 @@ static void cgwb_bdi_destroy(struct backing_dev_info *bdi)
        radix_tree_for_each_slot(slot, &bdi->cgwb_tree, &iter, 0)
                cgwb_kill(*slot);
 
-       rbtree_postorder_for_each_entry_safe(congested, congested_n,
-                                       &bdi->cgwb_congested_tree, rb_node) {
-               rb_erase(&congested->rb_node, &bdi->cgwb_congested_tree);
+       while ((rbn = rb_first(&bdi->cgwb_congested_tree))) {
+               struct bdi_writeback_congested *congested =
+                       rb_entry(rbn, struct bdi_writeback_congested, rb_node);
+
+               rb_erase(rbn, &bdi->cgwb_congested_tree);
                congested->bdi = NULL;  /* mark @congested unlinked */
        }