diff --git a/tests/io/081-flock_non_blocking_event_loop.phpt b/tests/io/081-flock_non_blocking_event_loop.phpt
index 7951708..f8c7dc3 100644
--- a/tests/io/081-flock_non_blocking_event_loop.phpt
+++ b/tests/io/081-flock_non_blocking_event_loop.phpt
@@ -29,7 +29,7 @@ $worker = spawn(function() {
     echo "worker: running\n";
 });
 
-// Let coroutines run — worker should complete, locker should be blocked in thread pool
+// Let coroutines run — worker should complete, locker should still be waiting for the lock
 Async\delay(50);
 
 echo "main: unlocking\n";
diff --git a/tests/io/084-flock_waiters_exceed_threadpool.phpt b/tests/io/084-flock_waiters_exceed_threadpool.phpt
new file mode 100644
index 0000000..a3d472a
--- /dev/null
+++ b/tests/io/084-flock_waiters_exceed_threadpool.phpt
@@ -0,0 +1,68 @@
+--TEST--
+More coroutines waiting for one file lock than the thread pool has threads
+--DESCRIPTION--
+Eight coroutines lock one file with the libuv thread pool pinned to four threads: each
+writes one line while holding the lock, and all eight lines must appear. A waiter must not
+occupy a pool thread. Reads and writes of a regular file are uv_fs requests on that same
+pool, so waiters that fill it leave the lock holder's own write with no thread to run on,
+and the lock is never released. A regression therefore shows as a hang with no output at
+all, not as a wrong count of lines. The lock itself is checked too: every waiter fails
+loudly if flock() returns false, and each compares the file size across its own suspension
+so that a lock granted to two coroutines at once is reported rather than appended over.
+true-async/php-async#221.
+--ENV--
+UV_THREADPOOL_SIZE=4
+--FILE--
+<?php
+
+use function Async\await_all;
+use function Async\delay;
+use function Async\spawn;
+
+const WAITERS = 8;
+
+$path = __DIR__ . '/084-flock.tmp';
+@unlink($path);
+
+$tasks = [];
+
+for ($i = 0; $i < WAITERS; $i++) {
+    $tasks[] = spawn(function () use ($i, $path) {
+        $fh = fopen($path, 'a');
+
+        if (!flock($fh, LOCK_EX)) {
+            echo "waiter $i: flock failed\n";
+            return;
+        }
+
+        // Held across a suspension point, so that the waiters really do overlap:
+        // without it each coroutine could take and drop the lock before the next runs.
+        clearstatcache(true, $path);
+        $size = filesize($path);
+        delay(1);
+        clearstatcache(true, $path);
+
+        if (filesize($path) !== $size) {
+            echo "waiter $i: another writer while the lock was held\n";
+        }
+
+        fwrite($fh, "waiter $i\n");
+        fflush($fh);
+
+        flock($fh, LOCK_UN);
+        fclose($fh);
+    });
+}
+
+await_all($tasks);
+
+echo "lines: ", count(file($path)), "\n";
+echo "ok\n";
+?>
+--CLEAN--
+<?php
+@unlink(__DIR__ . '/084-flock.tmp');
+?>
+--EXPECT--
+lines: 8
+ok