diff --git a/packages/react-reconciler/src/ReactFiberRootScheduler.js b/packages/react-reconciler/src/ReactFiberRootScheduler.js
index 8e0c65b855..835e9f9a70 100644
--- a/packages/react-reconciler/src/ReactFiberRootScheduler.js
+++ b/packages/react-reconciler/src/ReactFiberRootScheduler.js
@@ -151,29 +151,17 @@ export function flushSyncWorkOnLegacyRootsOnly() {
   flushSyncWorkAcrossRoots_impl(true);
 }
 
-function flushSyncWorkAcrossRoots_impl(onlyLegacy: boolean) {
-  if (isFlushingWork) {
-    // Prevent reentrancy.
-    // TODO: Is this overly defensive? The callers must check the execution
-    // context first regardless.
-    return;
-  }
-
-  if (!mightHavePendingSyncWork) {
-    // Fast path. There's no sync work to do.
-    return;
-  }
-
-  const workInProgressRoot = getWorkInProgressRoot();
-  const workInProgressRootRenderLanes = getWorkInProgressRootRenderLanes();
-
-  // There may or may not be synchronous work scheduled. Let's check.
-  let didPerformSomeWork;
+export function _doFlushWork(
+  firstRoot,
+  workInProgressRoot,
+  workInProgressRootRenderLanes,
+  onlyLegacy,
+) {
+  let didPerformSomeWork = false;
   let errors: Array<mixed> | null = null;
-  isFlushingWork = true;
   do {
     didPerformSomeWork = false;
-    let root = firstScheduledRoot;
+    let root = firstRoot;
     while (root !== null) {
       if (onlyLegacy && root.tag !== LegacyRoot) {
         // Skip non-legacy roots.
@@ -202,6 +190,33 @@ function flushSyncWorkAcrossRoots_impl(onlyLegacy: boolean) {
       root = root.next;
     }
   } while (didPerformSomeWork);
+
+  return errors;
+}
+function flushSyncWorkAcrossRoots_impl(onlyLegacy: boolean) {
+  if (isFlushingWork) {
+    // Prevent reentrancy.
+    // TODO: Is this overly defensive? The callers must check the execution
+    // context first regardless.
+    return;
+  }
+
+  if (!mightHavePendingSyncWork) {
+    // Fast path. There's no sync work to do.
+    return;
+  }
+
+  const workInProgressRoot = getWorkInProgressRoot();
+  const workInProgressRootRenderLanes = getWorkInProgressRootRenderLanes();
+
+  // There may or may not be synchronous work scheduled. Let's check.
+  isFlushingWork = true;
+  const errors = _doFlushWork(
+    firstScheduledRoot,
+    workInProgressRoot,
+    workInProgressRootRenderLanes,
+    onlyLegacy,
+  );
   isFlushingWork = false;
 
   // If any errors were thrown, rethrow them right before exiting.
diff --git a/packages/react-reconciler/src/__tests__/ReactFiberRootScheduler-test.js b/packages/react-reconciler/src/__tests__/ReactFiberRootScheduler-test.js
new file mode 100644
index 0000000000..1e6bf432e1
--- /dev/null
+++ b/packages/react-reconciler/src/__tests__/ReactFiberRootScheduler-test.js
@@ -0,0 +1,63 @@
+let _doFlushWork;
+const shimHostConfigPath = 'react-reconciler/src/ReactFiberConfig';
+
+jest.mock(shimHostConfigPath, () => {
+  return jest.requireActual(
+    'react-dom-bindings/src/client/ReactFiberConfigDOM.js',
+  );
+});
+beforeAll(() => {
+  _doFlushWork = require('../ReactFiberRootScheduler')._doFlushWork;
+});
+
+test('does not hang', () => {
+  const root = {
+    tag: 1,
+    pendingChildren: null,
+    pingCache: {},
+    finishedWork: null,
+    timeoutHandle: -1,
+    cancelPendingCommit: null,
+    context: {},
+    pendingContext: null,
+    next: null,
+    callbackNode: null,
+    callbackPriority: 0,
+    expirationTimes: [
+      -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 278303.90000000596,
+      278417.1999999881, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+      -1, -1, -1, -1,
+    ],
+    pendingLanes: 6176,
+    suspendedLanes: 0,
+    pingedLanes: 0,
+    expiredLanes: 0,
+    mutableReadLanes: 0,
+    finishedLanes: 0,
+    errorRecoveryDisabledLanes: 0,
+    entangledLanes: 6144,
+    entanglements: [
+      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 6144, 6144, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+      0, 0, 0, 0, 0, 0, 0, 0,
+    ],
+    hiddenUpdates: [],
+    identifierPrefix: '',
+    pooledCache: null,
+    pooledCacheLanes: 0,
+    mutableSourceEagerHydrationData: null,
+    hydrationCallbacks: {
+      unstable_concurrentUpdatesByDefault: true,
+      unstable_strictMode: true,
+    },
+    incompleteTransitions: {},
+    effectDuration: 0,
+    passiveEffectDuration: 0,
+    memoizedUpdaters: {},
+    pendingUpdatersLaneMap: [],
+    _debugRootType: 'hydrateRoot()',
+  };
+
+  expect(() => {
+    _doFlushWork(root, root, 2, false);
+  }).not.toThrow();
+});