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bug#58639: 29.0.50; [noverlay] Nested overlay iteration in GC
From: |
Stefan Monnier |
Subject: |
bug#58639: 29.0.50; [noverlay] Nested overlay iteration in GC |
Date: |
Wed, 19 Oct 2022 13:29:02 -0400 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/29.0.50 (gnu/linux) |
> Stefan, looks like recursive calls to 'mark_buffer' are not only
> possible but common. For this to be a problem for overlays, all it
> takes is an overlay property to somehow reference a second buffer.
> Boom, you get nested overlay iteration.
Duh, of course!
> Two easy fixes I can think of:
> a) Break the recursion while marking with a queue, rather than a stack.
> To bound the size of the queue do this only for buffers.
Actually, the alloc.c code already has that, so it's a trivial change.
> b) Add a specialized itree iteration function for gc. In fact,
> properties already have one that we can copy:
>
> extern void traverse_intervals_noorder (
> INTERVAL,
> void (*) (INTERVAL, void *), void *);
We even have better: just undo commit b8fbd42f0a7caa4cd9e2d50dd4e4b2101ac78acd
Or use the (100% guaranteed untested) code below.
The good thing about using `ITREE_FOREACH` in the GC is that it checks
that we're not running the GC from within an `ITREE_FOREACH` loop.
Side comment about `traverse_intervals_noorder`: I added this function
back when I played with using splay trees for `intervals.c`, which
worked well except that it occasionally created "pathologically" deep
trees, hence the need for `traverse_intervals_noorder` to keep the
recursion depth in O(log N).
> I like (b) so much I'll work on it now.
Fine by me :-)
Stefan
diff --git a/src/alloc.c b/src/alloc.c
index 00f2991f250..555df997dfb 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -6531,9 +6531,13 @@ mark_buffer (struct buffer *buffer)
if (!BUFFER_LIVE_P (buffer))
mark_object (BVAR (buffer, undo_list));
- struct interval_node *node;
- ITREE_FOREACH (node, buffer->overlays, PTRDIFF_MIN, PTRDIFF_MAX, ASCENDING)
- mark_object (node->data);
+ {
+ ptrdiff_t sp = mark_stk.sp;
+ struct interval_node *node;
+ ITREE_FOREACH (node, buffer->overlays, PTRDIFF_MIN, PTRDIFF_MAX, ASCENDING)
+ mark_stack_push_value (node->data);
+ process_mark_stack (sp);
+ }
/* If this is an indirect buffer, mark its base buffer. */
if (buffer->base_buffer &&