From e7d02a076a11a8732b364f0b6c34c6a1d246c1e8 Mon Sep 17 00:00:00 2001
From: Andrey Kolkov <a.kolkov@gmail.com>
Date: Thu, 30 Jul 2026 15:12:52 +0300
Subject: [PATCH] =?UTF-8?q?fix:=20Queue.LastSubmissionIndex=20deadlock=20?=
 =?UTF-8?q?=E2=80=94=20atomic=20instead=20of=20mutex=20(ADR-056)?=
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onZero callbacks from Triage() called lastSubmissionIndex() which locked
Queue.mu, already held by Submit(). Deadlock chain:
Submit → postSubmit → Triage → onZero → lastSubmissionIndex → Queue.mu

Fix: lastSubmissionIndex changed from mutex-protected uint64 to atomic.Uint64.
Single writer (Submit under mu), lock-free readers (onZero callbacks).
Safe because Store happens under mu (sequential) and Load is always-valid.
---
 CHANGELOG.md    | 10 ++++++++++
 queue_native.go | 19 ++++++++++---------
 2 files changed, 20 insertions(+), 9 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8866164e..711931ab 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,16 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [0.30.29] - 2026-07-30
+
+### Fixed
+
+- **Queue.LastSubmissionIndex deadlock** — `onZero` callbacks from `Triage()`
+  called `lastSubmissionIndex()` which locked `Queue.mu`, already held by
+  `Submit()`. Changed `lastSubmissionIndex` from `mutex`-protected `uint64` to
+  `atomic.Uint64` — single writer (Submit), lock-free readers (onZero callbacks).
+  ADR-056 deadlock chain: Submit→Triage→onZero→lastSubmissionIndex→mu.
+
 ## [0.30.28] - 2026-07-30
 
 ### Fixed
diff --git a/queue_native.go b/queue_native.go
index 9ae31ed5..6cc9e334 100644
--- a/queue_native.go
+++ b/queue_native.go
@@ -5,6 +5,7 @@ package wgpu
 import (
 	"fmt"
 	"sync"
+	"sync/atomic"
 
 	"github.com/gogpu/wgpu/core"
 	"github.com/gogpu/wgpu/hal"
@@ -27,8 +28,10 @@ type Queue struct {
 	// lastSubmissionIndex is the most recent submission index returned by
 	// hal.Queue.Submit(). Used by DestroyQueue to conservatively defer
 	// resource destruction until after the latest known submission completes.
-	// Protected by mu.
-	lastSubmissionIndex uint64
+	// Atomic: written by Submit (under mu), read by onZero callbacks which
+	// may fire during Triage while Queue.mu is held. Using atomic avoids
+	// deadlock: Submit→Triage→onZero→lastSubmissionIndex→mu (ADR-056).
+	lastSubmissionIndex atomic.Uint64
 }
 
 // Submit submits command buffers for execution. Non-blocking.
@@ -97,7 +100,7 @@ func (q *Queue) Submit(commandBuffers ...*CommandBuffer) (uint64, error) {
 	}
 
 	// Track the latest submission index for deferred resource destruction.
-	q.lastSubmissionIndex = subIdx
+	q.lastSubmissionIndex.Store(subIdx)
 
 	// Record inflight resources and clean up completed ones.
 	// dstTextures/dstBuffers prevent premature Release (BUG-DX12-006: use-after-free).
@@ -346,13 +349,11 @@ func (q *Queue) SetSwapchainSuppressed(suppressed bool) {
 }
 
 // LastSubmissionIndex returns the most recent submission index.
-// Used by resource Release() methods to schedule deferred destruction.
-// Safe for concurrent use — reads under the queue mutex.
+// Used by resource Release() and onZero callbacks to schedule deferred destruction.
+// Lock-free via atomic — safe to call from onZero callbacks during Triage
+// while Queue.mu is held by Submit. ADR-056 deadlock fix.
 func (q *Queue) LastSubmissionIndex() uint64 {
-	q.mu.Lock()
-	idx := q.lastSubmissionIndex
-	q.mu.Unlock()
-	return idx
+	return q.lastSubmissionIndex.Load()
 }
 
 // destroyQueue returns the device's DestroyQueue, or nil if unavailable.