LATX, fix: Preserve queued RCU callbacks across fork

tests/README.md

@@ -138,3 +138,35 @@ Before submitting a new test target, verify both of these:
 
 1. A normal product build without `--enable-tests` does not build the test.
 2. A build configured with `--enable-tests` builds and runs the selected suite.
+
+## RCU fork queue regression
+
+`test-rcu-fork-queue` runs on POSIX hosts with `--enable-tests`, including
+non-KZT and non-LATX configurations. It covers empty forks, callbacks that
+queue another callback, and forks with concurrently published callbacks still
+queued. A blocking callback pauses the consumer outside the grace-period lock,
+so the ordinary atfork lock path can proceed. The blocker itself has already
+been dequeued and is outside the queue-preservation contract.
+
+LATX/KZT configurations also register `test-rcu-fork-queue-held-reader`, which
+holds a reader across fork and covers lazy and deferred child startup. This
+scenario depends on KZT's atfork path not waiting for the parent's grace period.
+Both cases check each callback marked published after `call_rcu1()` returns:
+after the child drains, every published callback in its snapshot must have run
+exactly once. A publisher remains active during the forks, and the parent still
+checks all 512 callbacks. Child drains run one at a time and the test is
+scheduled before other suite entries because QEMU user-mode can strand a
+post-fork worker startup when several emulated threaded-fork workloads compete;
+this scheduling does not relax the per-callback assertions.
+
+Run the common case explicitly (the binary is not part of the default build):
+
+```sh
+meson test -C build64-tests test-rcu-fork-queue --print-errorlogs
+```
+
+With LATX/KZT enabled, also run:
+
+```sh
+meson test -C build64-tests test-rcu-fork-queue-held-reader --print-errorlogs
+```

tests/unit/meson.build

@@ -169,6 +169,25 @@ test(
   suite: 'lat-pr-fast',
 )
 
+if 'CONFIG_POSIX' in config_host
+  test_rcu_fork_queue = executable(
+    'test-rcu-fork-queue',
+    files('test-rcu-fork-queue.c') + genh,
+    include_directories: include_directories('../..'),
+    dependencies: [glib, qemuutil],
+    build_by_default: false,
+  )
+  # QEMU user-mode can strand post-fork worker startup after several emulated
+  # tests have competed.  Run this fork/thread stress test first and isolated.
+  test('test-rcu-fork-queue', test_rcu_fork_queue,
+       suite: 'lat-pr-fast', timeout: 45, is_parallel: false, priority: 100)
+  if 'CONFIG_LATX' in config_host and 'CONFIG_LATX_KZT' in config_host
+    test('test-rcu-fork-queue-held-reader', test_rcu_fork_queue,
+         args: ['--held-reader'], suite: 'lat-pr-fast', timeout: 45,
+         is_parallel: false, priority: 100)
+  endif
+endif
+
 if 'CONFIG_LATX' in config_host
   test_x11_async_handler = executable(
     'test-x11-async-handler',