From 30ddd43e6da80effba4b0061dc9a8196f9a7068f Mon Sep 17 00:00:00 2001
From: Karol Herbst <kherbst@redhat.com>
Date: Tue, 14 May 2024 12:38:14 +0200
Subject: [PATCH] event: break long dependency chains on drop

This prevents stack overflows on drop without making it expensive to read
from dependencies (e.g. my attempt to use Weak instead).

Cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29190>
(cherry picked from commit 48c752d3e060fc62dcc5ed228c2464efc6591029)
---
 .pick_status.json                           |  2 +-
 src/gallium/frontends/rusticl/core/event.rs | 22 +++++++++++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/.pick_status.json b/.pick_status.json
index 580b517933b..725cb9dff59 100644
--- a/.pick_status.json
+++ b/.pick_status.json
@@ -2264,7 +2264,7 @@
         "description": "event: break long dependency chains on drop",
         "nominated": true,
         "nomination_type": 0,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": null,
         "notes": null
diff --git a/src/gallium/frontends/rusticl/core/event.rs b/src/gallium/frontends/rusticl/core/event.rs
index 6389d6de831..bc0849aab93 100644
--- a/src/gallium/frontends/rusticl/core/event.rs
+++ b/src/gallium/frontends/rusticl/core/event.rs
@@ -10,6 +10,7 @@ use mesa_rust_util::static_assert;
 use rusticl_opencl_gen::*;
 
 use std::collections::HashSet;
+use std::mem;
 use std::slice;
 use std::sync::Arc;
 use std::sync::Condvar;
@@ -272,6 +273,27 @@ impl Event {
     }
 }
 
+impl Drop for Event {
+    // implement drop in order to prevent stack overflows of long dependency chains.
+    //
+    // This abuses the fact that `Arc::into_inner` only succeeds when there is one strong reference
+    // so we turn a recursive drop chain into a drop list for events having no other references.
+    fn drop(&mut self) {
+        if self.deps.is_empty() {
+            return;
+        }
+
+        let mut deps_list = vec![mem::take(&mut self.deps)];
+        while let Some(deps) = deps_list.pop() {
+            for dep in deps {
+                if let Some(mut dep) = Arc::into_inner(dep) {
+                    deps_list.push(mem::take(&mut dep.deps));
+                }
+            }
+        }
+    }
+}
+
 // TODO worker thread per device
 // Condvar to wait on new events to work on
 // notify condvar when flushing queue events to worker