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author | Cody P Schafer <dev@codyps.com> | 2015-11-06 16:31:28 -0800 |
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committer | Linus Torvalds <torvalds@linux-foundation.org> | 2015-11-06 17:50:42 -0800 |
commit | 8de1ee7ebfb4979c6444e81273e12e7a972c367d (patch) | |
tree | a1d45a54168a77a814537a50cc29cc4fdcf35615 | |
parent | 90224350eaaf8b8043b19c393048f732bc2e4120 (diff) | |
download | linux-8de1ee7ebfb4979c6444e81273e12e7a972c367d.tar.bz2 |
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 <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>
-rw-r--r-- | include/linux/rbtree.h | 12 |
1 files 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); \ |