diff --git a/scripts/fiber/tests-passing.txt b/scripts/fiber/tests-passing.txt
index c56a44b5b28..c749ed37ab6 100644
--- a/scripts/fiber/tests-passing.txt
+++ b/scripts/fiber/tests-passing.txt
@@ -1207,6 +1207,7 @@ src/renderers/shared/fiber/__tests__/ReactIncrementalScheduling-test.js
 * can opt-in to deferred/animation scheduling inside componentDidMount/Update
 * performs Task work even after time runs out
 * does not perform animation work after time runs out
+* can force synchronous updates with syncUpdates, even inside batchedUpdates
 
 src/renderers/shared/fiber/__tests__/ReactIncrementalSideEffects-test.js
 * can update child nodes of a host instance
@@ -1576,6 +1577,7 @@ src/renderers/shared/shared/__tests__/ReactUpdates-test.js
 * unstable_batchedUpdates should return value from a callback
 * unmounts and remounts a root in the same batch
 * handles reentrant mounting in synchronous mode
+* mounts and unmounts are sync even in a batch
 
 src/renderers/shared/shared/__tests__/refs-destruction-test.js
 * should remove refs when destroying the parent
diff --git a/src/renderers/dom/fiber/ReactDOMFiber.js b/src/renderers/dom/fiber/ReactDOMFiber.js
index 20d851dcee6..26c37349098 100644
--- a/src/renderers/dom/fiber/ReactDOMFiber.js
+++ b/src/renderers/dom/fiber/ReactDOMFiber.js
@@ -67,9 +67,10 @@ type HostContextDev = {
 };
 type HostContextProd = string;
 type HostContext = HostContextDev | HostContextProd;
-
-let eventsEnabled : ?boolean = null;
-let selectionInformation : ?mixed = null;
+type CommitInfo = {
+  eventsEnabled: boolean,
+  selectionInformation: mixed,
+};
 
 var ELEMENT_NODE_TYPE = 1;
 var DOC_NODE_TYPE = 9;
@@ -137,17 +138,18 @@ var DOMRenderer = ReactFiberReconciler({
     return getChildNamespace(parentNamespace, type);
   },
 
-  prepareForCommit() : void {
-    eventsEnabled = ReactBrowserEventEmitter.isEnabled();
+  prepareForCommit() : CommitInfo {
+    const eventsEnabled = ReactBrowserEventEmitter.isEnabled();
     ReactBrowserEventEmitter.setEnabled(false);
-    selectionInformation = ReactInputSelection.getSelectionInformation();
+    return {
+      eventsEnabled,
+      selectionInformation: ReactInputSelection.getSelectionInformation(),
+    };
   },
 
-  resetAfterCommit() : void {
-    ReactInputSelection.restoreSelection(selectionInformation);
-    selectionInformation = null;
-    ReactBrowserEventEmitter.setEnabled(eventsEnabled);
-    eventsEnabled = null;
+  resetAfterCommit(commitInfo : CommitInfo) : void {
+    ReactInputSelection.restoreSelection(commitInfo.selectionInformation);
+    ReactBrowserEventEmitter.setEnabled(commitInfo.eventsEnabled);
   },
 
   createInstance(
@@ -322,9 +324,15 @@ function renderSubtreeIntoContainer(parentComponent : ?ReactComponent<any, any,
     while (container.lastChild) {
       container.removeChild(container.lastChild);
     }
-    root = container._reactRootContainer = DOMRenderer.createContainer(container);
+    const newRoot = DOMRenderer.createContainer(container);
+    root = container._reactRootContainer = newRoot;
+    // Initial mount is always sync, even if we're in a batch.
+    DOMRenderer.syncUpdates(() => {
+      DOMRenderer.updateContainer(children, newRoot, parentComponent, callback);
+    });
+  } else {
+    DOMRenderer.updateContainer(children, root, parentComponent, callback);
   }
-  DOMRenderer.updateContainer(children, root, parentComponent, callback);
   return DOMRenderer.getPublicRootInstance(root);
 }
 
@@ -346,8 +354,11 @@ var ReactDOM = {
   unmountComponentAtNode(container : DOMContainerElement) {
     warnAboutUnstableUse();
     if (container._reactRootContainer) {
-      return renderSubtreeIntoContainer(null, null, container, () => {
-        container._reactRootContainer = null;
+      // Unmount is always sync, even if we're in a batch.
+      return DOMRenderer.syncUpdates(() => {
+        return renderSubtreeIntoContainer(null, null, container, () => {
+          container._reactRootContainer = null;
+        });
       });
     }
   },
diff --git a/src/renderers/shared/__tests__/ReactPerf-test.js b/src/renderers/shared/__tests__/ReactPerf-test.js
index b8f895fc883..2695e648b2b 100644
--- a/src/renderers/shared/__tests__/ReactPerf-test.js
+++ b/src/renderers/shared/__tests__/ReactPerf-test.js
@@ -16,6 +16,7 @@ describe('ReactPerf', () => {
   var ReactDOM;
   var ReactPerf;
   var ReactTestUtils;
+  var ReactDOMFeatureFlags;
   var emptyFunction;
 
   var App;
@@ -38,6 +39,7 @@ describe('ReactPerf', () => {
     ReactDOM = require('ReactDOM');
     ReactPerf = require('ReactPerf');
     ReactTestUtils = require('ReactTestUtils');
+    ReactDOMFeatureFlags = require('ReactDOMFeatureFlags');
     emptyFunction = require('emptyFunction');
 
     App = class extends React.Component {
@@ -614,7 +616,7 @@ describe('ReactPerf', () => {
       }
     }
     class EvilPortal extends React.Component {
-      componentWillMount() {
+      componentDidMount() {
         var portalContainer = document.createElement('div');
         ReactDOM.render(<Evil />, portalContainer);
       }
@@ -645,6 +647,10 @@ describe('ReactPerf', () => {
     var container = document.createElement('div');
     var thrownErr = new Error('Muhaha!');
 
+    if (ReactDOMFeatureFlags.useFiber) {
+      spyOn(console, 'error');
+    }
+
     class Evil extends React.Component {
       componentWillMount() {
         throw thrownErr;
@@ -679,6 +685,15 @@ describe('ReactPerf', () => {
     }
     ReactDOM.unmountComponentAtNode(container);
     ReactPerf.stop();
+
+    if (ReactDOMFeatureFlags.useFiber) {
+      // A sync `render` inside cWM will print a warning. That should be the
+      // only warning.
+      expect(console.error.calls.count()).toEqual(1);
+      expect(console.error.calls.argsFor(0)[0]).toMatch(
+        /Render methods should be a pure function of props and state/
+      );
+    }
   });
 
   it('should not print errant warnings if portal throws in componentDidMount()', () => {
@@ -694,7 +709,7 @@ describe('ReactPerf', () => {
       }
     }
     class EvilPortal extends React.Component {
-      componentWillMount() {
+      componentDidMount() {
         var portalContainer = document.createElement('div');
         ReactDOM.render(<Evil />, portalContainer);
       }
diff --git a/src/renderers/shared/fiber/ReactFiberBeginWork.js b/src/renderers/shared/fiber/ReactFiberBeginWork.js
index 4cfb1f4bb1b..e5ee7a831d1 100644
--- a/src/renderers/shared/fiber/ReactFiberBeginWork.js
+++ b/src/renderers/shared/fiber/ReactFiberBeginWork.js
@@ -66,8 +66,8 @@ if (__DEV__) {
   var ReactDebugCurrentFiber = require('ReactDebugCurrentFiber');
 }
 
-module.exports = function<T, P, I, TI, C, CX>(
-  config : HostConfig<T, P, I, TI, C, CX>,
+module.exports = function<T, P, I, TI, C, CX, CI>(
+  config : HostConfig<T, P, I, TI, C, CX, CI>,
   hostContext : HostContext<C, CX>,
   scheduleUpdate : (fiber : Fiber, priorityLevel : PriorityLevel) => void,
   getPriorityContext : () => PriorityLevel,
diff --git a/src/renderers/shared/fiber/ReactFiberCommitWork.js b/src/renderers/shared/fiber/ReactFiberCommitWork.js
index bb56a4c78f3..429e892e0be 100644
--- a/src/renderers/shared/fiber/ReactFiberCommitWork.js
+++ b/src/renderers/shared/fiber/ReactFiberCommitWork.js
@@ -33,8 +33,8 @@ var {
   ContentReset,
 } = require('ReactTypeOfSideEffect');
 
-module.exports = function<T, P, I, TI, C, CX>(
-  config : HostConfig<T, P, I, TI, C, CX>,
+module.exports = function<T, P, I, TI, C, CX, CI>(
+  config : HostConfig<T, P, I, TI, C, CX, CI>,
   hostContext : HostContext<C, CX>,
   captureError : (failedFiber : Fiber, error: Error) => ?Fiber
 ) {
diff --git a/src/renderers/shared/fiber/ReactFiberCompleteWork.js b/src/renderers/shared/fiber/ReactFiberCompleteWork.js
index bffecb3c469..8c836cfda5d 100644
--- a/src/renderers/shared/fiber/ReactFiberCompleteWork.js
+++ b/src/renderers/shared/fiber/ReactFiberCompleteWork.js
@@ -46,8 +46,8 @@ if (__DEV__) {
   var ReactDebugCurrentFiber = require('ReactDebugCurrentFiber');
 }
 
-module.exports = function<T, P, I, TI, C, CX>(
-  config : HostConfig<T, P, I, TI, C, CX>,
+module.exports = function<T, P, I, TI, C, CX, CI>(
+  config : HostConfig<T, P, I, TI, C, CX, CI>,
   hostContext : HostContext<C, CX>,
 ) {
   const {
diff --git a/src/renderers/shared/fiber/ReactFiberHostContext.js b/src/renderers/shared/fiber/ReactFiberHostContext.js
index 529271de6ad..de311124ca9 100644
--- a/src/renderers/shared/fiber/ReactFiberHostContext.js
+++ b/src/renderers/shared/fiber/ReactFiberHostContext.js
@@ -34,8 +34,8 @@ export type HostContext<C, CX> = {
   resetHostContainer() : void,
 };
 
-module.exports = function<T, P, I, TI, C, CX>(
-  config : HostConfig<T, P, I, TI, C, CX>
+module.exports = function<T, P, I, TI, C, CX, CI>(
+  config : HostConfig<T, P, I, TI, C, CX, CI>
 ) : HostContext<C, CX> {
   const {
     getChildHostContext,
diff --git a/src/renderers/shared/fiber/ReactFiberReconciler.js b/src/renderers/shared/fiber/ReactFiberReconciler.js
index ff55cbddaa0..3424ef683fa 100644
--- a/src/renderers/shared/fiber/ReactFiberReconciler.js
+++ b/src/renderers/shared/fiber/ReactFiberReconciler.js
@@ -43,7 +43,7 @@ export type Deadline = {
 
 type OpaqueNode = Fiber;
 
-export type HostConfig<T, P, I, TI, C, CX> = {
+export type HostConfig<T, P, I, TI, C, CX, CI> = {
 
   getRootHostContext(rootContainerInstance : C) : CX,
   getChildHostContext(parentHostContext : CX, type : T) : CX,
@@ -69,8 +69,8 @@ export type HostConfig<T, P, I, TI, C, CX> = {
   scheduleAnimationCallback(callback : () => void) : void,
   scheduleDeferredCallback(callback : (deadline : Deadline) => void) : void,
 
-  prepareForCommit() : void,
-  resetAfterCommit() : void,
+  prepareForCommit() : CI,
+  resetAfterCommit(commitInfo : CI) : void,
 
   useSyncScheduling ?: boolean,
 };
@@ -100,7 +100,7 @@ getContextForSubtree._injectFiber(function(fiber : Fiber) {
     parentContext;
 });
 
-module.exports = function<T, P, I, TI, C, CX>(config : HostConfig<T, P, I, TI, C, CX>) : Reconciler<C, I, TI> {
+module.exports = function<T, P, I, TI, C, CX, CI>(config : HostConfig<T, P, I, TI, C, CX, CI>) : Reconciler<C, I, TI> {
 
   var {
     scheduleUpdate,
diff --git a/src/renderers/shared/fiber/ReactFiberScheduler.js b/src/renderers/shared/fiber/ReactFiberScheduler.js
index 63cea47ee4c..20603ec6889 100644
--- a/src/renderers/shared/fiber/ReactFiberScheduler.js
+++ b/src/renderers/shared/fiber/ReactFiberScheduler.js
@@ -69,11 +69,12 @@ var {
 if (__DEV__) {
   var ReactFiberInstrumentation = require('ReactFiberInstrumentation');
   var ReactDebugCurrentFiber = require('ReactDebugCurrentFiber');
+  var warning = require('warning');
 }
 
 var timeHeuristicForUnitOfWork = 1;
 
-module.exports = function<T, P, I, TI, C, CX>(config : HostConfig<T, P, I, TI, C, CX>) {
+module.exports = function<T, P, I, TI, C, CX, CI>(config : HostConfig<T, P, I, TI, C, CX, CI>) {
   const hostContext = ReactFiberHostContext(config);
   const { popHostContainer, popHostContext, resetHostContainer } = hostContext;
   const { beginWork, beginFailedWork } = ReactFiberBeginWork(
@@ -107,10 +108,12 @@ module.exports = function<T, P, I, TI, C, CX>(config : HostConfig<T, P, I, TI, C
   // is thrown during reconciliation.
   let priorityContextBeforeReconciliation : PriorityLevel = NoWork;
 
-  // Keeps track of whether we're currently in a work loop. Used to batch
-  // nested updates.
+  // Keeps track of whether we're currently in a work loop.
   let isPerformingWork : boolean = false;
 
+  // Keeps track of whether sync updates should be downgraded to task updates.
+  let shouldDeferSyncUpdates : boolean = false;
+
   // The next work in progress fiber that we're currently working on.
   let nextUnitOfWork : ?Fiber = null;
   let nextPriorityLevel : PriorityLevel = NoWork;
@@ -345,7 +348,7 @@ module.exports = function<T, P, I, TI, C, CX>(config : HostConfig<T, P, I, TI, C
       firstEffect = finishedWork.firstEffect;
     }
 
-    prepareForCommit();
+    const commitInfo = prepareForCommit();
 
     // Commit all the side-effects within a tree. We'll do this in two passes.
     // The first pass performs all the host insertions, updates, deletions and
@@ -366,7 +369,7 @@ module.exports = function<T, P, I, TI, C, CX>(config : HostConfig<T, P, I, TI, C
       }
     }
 
-    resetAfterCommit();
+    resetAfterCommit(commitInfo);
 
     // In the second pass we'll perform all life-cycles and ref callbacks.
     // Life-cycles happen as a separate pass so that all placements, updates,
@@ -672,10 +675,31 @@ module.exports = function<T, P, I, TI, C, CX>(config : HostConfig<T, P, I, TI, C
   }
 
   function performWork(priorityLevel : PriorityLevel, deadline : Deadline | null) {
-    if (isPerformingWork) {
-      throw new Error('performWork was called recursively.');
-    }
+    // If performWork is called recursively, we need to save the previous state
+    // of the scheduler so it can be restored before the function exits.
+    // Recursion is only possible when using syncUpdates.
+    const previousPriorityContext = priorityContext;
+    const previousPriorityContextBeforeReconciliation = priorityContextBeforeReconciliation;
+    const previousIsPerformingWork = isPerformingWork;
+    const previousShouldDeferSyncUpdates = shouldDeferSyncUpdates;
+    const previousNextEffect = nextEffect;
+    const previousCommitPhaseBoundaries = commitPhaseBoundaries;
+    const previousFirstUncaughtError = firstUncaughtError;
+    const previousFatalError = fatalError;
+    const previousIsCommitting = isCommitting;
+    const previousIsUnmounting = isUnmounting;
+
+    priorityContext = NoWork;
+    priorityContextBeforeReconciliation = NoWork;
     isPerformingWork = true;
+    shouldDeferSyncUpdates = true;
+    nextEffect = null;
+    commitPhaseBoundaries = null;
+    firstUncaughtError = null;
+    fatalError = null;
+    isCommitting = false;
+    isUnmounting = false;
+
     const isPerformingDeferredWork = Boolean(deadline);
     let deadlineHasExpired = false;
 
@@ -705,39 +729,40 @@ module.exports = function<T, P, I, TI, C, CX>(config : HostConfig<T, P, I, TI, C
       } catch (error) {
         // We caught an error during either the begin or complete phases.
         const failedWork = nextUnitOfWork;
-        if (!failedWork) {
-          throw new Error('Should have nextUnitOfWork.');
-        }
-
-        // Reset the priority context to its value before reconcilation.
-        priorityContext = priorityContextBeforeReconciliation;
-
-        // "Capture" the error by finding the nearest boundary. If there is no
-        // error boundary, the nearest host container acts as one. If
-        // captureError returns null, the error was intentionally ignored.
-        const maybeBoundary = captureError(failedWork, error);
-        if (maybeBoundary) {
-          const boundary = maybeBoundary;
-
-          // Complete the boundary as if it rendered null. This will unmount
-          // the failed tree.
-          beginFailedWork(boundary.alternate, boundary, priorityLevel);
-
-          // The next unit of work is now the boundary that captured the error.
-          // Conceptually, we're unwinding the stack. We need to unwind the
-          // context stack, too, from the failed work to the boundary that
-          // captured the error.
-          // TODO: If we set the memoized props in beginWork instead of
-          // completeWork, rather than unwind the stack, we can just restart
-          // from the root. Can't do that until then because without memoized
-          // props, the nodes higher up in the tree will rerender unnecessarily.
-          if (failedWork) {
+        if (failedWork) {
+          // Reset the priority context to its value before reconcilation.
+          priorityContext = priorityContextBeforeReconciliation;
+
+          // "Capture" the error by finding the nearest boundary. If there is no
+          // error boundary, the nearest host container acts as one. If
+          // captureError returns null, the error was intentionally ignored.
+          const maybeBoundary = captureError(failedWork, error);
+          if (maybeBoundary) {
+            const boundary = maybeBoundary;
+
+            // Complete the boundary as if it rendered null. This will unmount
+            // the failed tree.
+            beginFailedWork(boundary.alternate, boundary, priorityLevel);
+
+            // The next unit of work is now the boundary that captured the error.
+            // Conceptually, we're unwinding the stack. We need to unwind the
+            // context stack, too, from the failed work to the boundary that
+            // captured the error.
+            // TODO: If we set the memoized props in beginWork instead of
+            // completeWork, rather than unwind the stack, we can just restart
+            // from the root. Can't do that until then because without memoized
+            // props, the nodes higher up in the tree will rerender unnecessarily.
             unwindContexts(failedWork, boundary);
+            nextUnitOfWork = completeUnitOfWork(boundary);
           }
-          nextUnitOfWork = completeUnitOfWork(boundary);
+          // Continue performing work
+          continue;
+        } else if (!fatalError) {
+          // There is no current unit of work. This is a worst-case scenario
+          // and should only be possible if there's a bug in the renderer, e.g.
+          // inside resetAfterCommit.
+          fatalError = error;
         }
-        // Continue performing work
-        continue;
       } finally {
         priorityContext = priorityContextBeforeReconciliation;
       }
@@ -778,22 +803,23 @@ module.exports = function<T, P, I, TI, C, CX>(config : HostConfig<T, P, I, TI, C
       }
     }
 
-    // We're done performing work. Time to clean up.
-    isPerformingWork = false;
-    capturedErrors = null;
-    failedBoundaries = null;
-
-    // It's now safe to throw errors.
-    if (fatalError) {
-      let e = fatalError;
-      fatalError = null;
-      firstUncaughtError = null;
-      throw e;
-    }
-    if (firstUncaughtError) {
-      let e = firstUncaughtError;
-      firstUncaughtError = null;
-      throw e;
+    const errorToThrow = fatalError || firstUncaughtError;
+
+    // We're done performing work. Restore the previous state of the scheduler.
+    priorityContext = previousPriorityContext;
+    priorityContextBeforeReconciliation = previousPriorityContextBeforeReconciliation;
+    isPerformingWork = previousIsPerformingWork;
+    shouldDeferSyncUpdates = previousShouldDeferSyncUpdates;
+    nextEffect = previousNextEffect;
+    commitPhaseBoundaries = previousCommitPhaseBoundaries;
+    firstUncaughtError = previousFirstUncaughtError;
+    fatalError = previousFatalError;
+    isCommitting = previousIsCommitting;
+    isUnmounting = previousIsUnmounting;
+
+    // It's safe to throw any unhandled errors.
+    if (errorToThrow) {
+      throw errorToThrow;
     }
   }
 
@@ -900,6 +926,8 @@ module.exports = function<T, P, I, TI, C, CX>(config : HostConfig<T, P, I, TI, C
   }
 
   function hasCapturedError(fiber : Fiber) : boolean {
+    // TODO: capturedErrors should store the boundary instance, to avoid needing
+    // to check the alternate.
     return Boolean(
       capturedErrors &&
       (capturedErrors.has(fiber) || (fiber.alternate && capturedErrors.has(fiber.alternate)))
@@ -907,11 +935,12 @@ module.exports = function<T, P, I, TI, C, CX>(config : HostConfig<T, P, I, TI, C
   }
 
   function isFailedBoundary(fiber : Fiber) : boolean {
-    const res = Boolean(
+    // TODO: failedBoundaries should store the boundary instance, to avoid
+    // needing to check the alternate.
+    return Boolean(
       failedBoundaries &&
       (failedBoundaries.has(fiber) || (fiber.alternate && failedBoundaries.has(fiber.alternate)))
     );
-    return res;
   }
 
   function commitErrorHandling(effectfulFiber : Fiber) {
@@ -1001,8 +1030,17 @@ module.exports = function<T, P, I, TI, C, CX>(config : HostConfig<T, P, I, TI, C
   }
 
   function scheduleUpdate(fiber : Fiber, priorityLevel : PriorityLevel) {
-    // If we're in a batch, downgrade sync priority to task priority
-    if (priorityLevel === SynchronousPriority && isPerformingWork) {
+    // Detect if a synchronous update is made during render (or begin phase).
+    if (priorityLevel === SynchronousPriority && isPerformingWork && !isCommitting) {
+      if (__DEV__) {
+        warning(
+          false,
+          'Render methods should be a pure function of props and state; ' +
+          'triggering nested component updates from render is not allowed. ' +
+          'If necessary, trigger nested updates in componentDidUpdate.'
+        );
+      }
+      // Downgrade to Task priority to prevent an infinite loop.
       priorityLevel = TaskPriority;
     }
 
@@ -1035,15 +1073,11 @@ module.exports = function<T, P, I, TI, C, CX>(config : HostConfig<T, P, I, TI, C
           // schedule a callback to perform work later.
           switch (priorityLevel) {
             case SynchronousPriority:
-              // Perform work immediately
               performWork(SynchronousPriority);
               return;
             case TaskPriority:
-              // If we're already performing work, Task work will be flushed before
-              // exiting the current batch. So we can skip it here.
-              if (!isPerformingWork) {
-                performWork(TaskPriority);
-              }
+              // TODO: If we're not already performing work, schedule a
+              // deferred callback.
               return;
             case AnimationPriority:
               scheduleAnimationCallback(performAnimationWork);
@@ -1064,7 +1098,8 @@ module.exports = function<T, P, I, TI, C, CX>(config : HostConfig<T, P, I, TI, C
   }
 
   function getPriorityContext() : PriorityLevel {
-    if (priorityContext === SynchronousPriority && isPerformingWork) {
+    // If we're in a batch, downgrade sync priority to task priority
+    if (priorityContext === SynchronousPriority && shouldDeferSyncUpdates) {
       return TaskPriority;
     }
     return priorityContext;
@@ -1085,16 +1120,15 @@ module.exports = function<T, P, I, TI, C, CX>(config : HostConfig<T, P, I, TI, C
   }
 
   function batchedUpdates<A, R>(fn : (a: A) => R, a : A) : R {
-    const previousIsPerformingWork = isPerformingWork;
-    // Simulate that we're performing work so that sync work is batched
-    isPerformingWork = true;
+    const previousShouldDeferSyncUpdates = shouldDeferSyncUpdates;
+    shouldDeferSyncUpdates = true;
     try {
       return fn(a);
     } finally {
-      isPerformingWork = previousIsPerformingWork;
-      // If we're not already performing work, we need to flush any task work
+      shouldDeferSyncUpdates = previousShouldDeferSyncUpdates;
+      // If we're not already inside a batch, we need to flush any task work
       // that was created by the user-provided function.
-      if (!isPerformingWork) {
+      if (!shouldDeferSyncUpdates) {
         performWork(TaskPriority);
       }
     }
@@ -1102,11 +1136,14 @@ module.exports = function<T, P, I, TI, C, CX>(config : HostConfig<T, P, I, TI, C
 
   function syncUpdates<A>(fn : () => A) : A {
     const previousPriorityContext = priorityContext;
+    const previousShouldDeferSyncUpdates = shouldDeferSyncUpdates;
     priorityContext = SynchronousPriority;
+    shouldDeferSyncUpdates = false;
     try {
       return fn();
     } finally {
       priorityContext = previousPriorityContext;
+      shouldDeferSyncUpdates = previousShouldDeferSyncUpdates;
     }
   }
 
diff --git a/src/renderers/shared/fiber/__tests__/ReactIncrementalScheduling-test.js b/src/renderers/shared/fiber/__tests__/ReactIncrementalScheduling-test.js
index 26e14b99bef..33a43df6d6f 100644
--- a/src/renderers/shared/fiber/__tests__/ReactIncrementalScheduling-test.js
+++ b/src/renderers/shared/fiber/__tests__/ReactIncrementalScheduling-test.js
@@ -340,4 +340,14 @@ describe('ReactIncrementalScheduling', () => {
     // animation priority.
     expect(ReactNoop.getChildren()).toEqual([span(1)]);
   });
+
+  it('can force synchronous updates with syncUpdates, even inside batchedUpdates', done => {
+    ReactNoop.batchedUpdates(() => {
+      ReactNoop.syncUpdates(() => {
+        ReactNoop.render(<span />);
+        expect(ReactNoop.getChildren()).toEqual([span()]);
+        done();
+      });
+    });
+  });
 });
diff --git a/src/renderers/shared/hooks/__tests__/ReactComponentTreeHook-test.js b/src/renderers/shared/hooks/__tests__/ReactComponentTreeHook-test.js
index db281517a49..e0114b56431 100644
--- a/src/renderers/shared/hooks/__tests__/ReactComponentTreeHook-test.js
+++ b/src/renderers/shared/hooks/__tests__/ReactComponentTreeHook-test.js
@@ -18,6 +18,7 @@ describe('ReactComponentTreeHook', () => {
   var ReactInstanceMap;
   var ReactComponentTreeHook;
   var ReactComponentTreeTestUtils;
+  var ReactDOMFeatureFlags;
 
   beforeEach(() => {
     jest.resetModules();
@@ -28,6 +29,7 @@ describe('ReactComponentTreeHook', () => {
     ReactInstanceMap = require('ReactInstanceMap');
     ReactComponentTreeHook = require('ReactComponentTreeHook');
     ReactComponentTreeTestUtils = require('ReactComponentTreeTestUtils');
+    ReactDOMFeatureFlags = require('ReactDOMFeatureFlags');
   });
 
   function assertTreeMatches(pairs) {
@@ -1841,6 +1843,9 @@ describe('ReactComponentTreeHook', () => {
       // https://github.com/facebook/react/issues/7187
       var el = document.createElement('div');
       var portalEl = document.createElement('div');
+      if (ReactDOMFeatureFlags.useFiber) {
+        spyOn(console, 'error');
+      }
       class Foo extends React.Component {
         componentWillMount() {
           ReactDOM.render(<div />, portalEl);
@@ -1850,6 +1855,14 @@ describe('ReactComponentTreeHook', () => {
         }
       }
       ReactDOM.render(<Foo />, el);
+      if (ReactDOMFeatureFlags.useFiber) {
+        // A sync `render` inside cWM will print a warning. That should be the
+        // only warning.
+        expect(console.error.calls.count()).toEqual(1);
+        expect(console.error.calls.argsFor(0)[0]).toMatch(
+          /Render methods should be a pure function of props and state/
+        );
+      }
     });
 
     it('is created when calling renderToString during render', () => {
diff --git a/src/renderers/shared/shared/__tests__/ReactCompositeComponent-test.js b/src/renderers/shared/shared/__tests__/ReactCompositeComponent-test.js
index 1a302d6fcac..af226d2f229 100644
--- a/src/renderers/shared/shared/__tests__/ReactCompositeComponent-test.js
+++ b/src/renderers/shared/shared/__tests__/ReactCompositeComponent-test.js
@@ -1266,7 +1266,7 @@ describe('ReactCompositeComponent', () => {
     var layer = document.createElement('div');
 
     class Component extends React.Component {
-      componentWillMount() {
+      componentDidMount() {
         ReactDOM.render(<div />, layer);
       }
 
diff --git a/src/renderers/shared/shared/__tests__/ReactUpdates-test.js b/src/renderers/shared/shared/__tests__/ReactUpdates-test.js
index 82113a46d31..638ab6eda26 100644
--- a/src/renderers/shared/shared/__tests__/ReactUpdates-test.js
+++ b/src/renderers/shared/shared/__tests__/ReactUpdates-test.js
@@ -1142,4 +1142,26 @@ describe('ReactUpdates', () => {
     expect(container.textContent).toBe('goodbye');
     expect(mounts).toBe(1);
   });
+
+  it('mounts and unmounts are sync even in a batch', () => {
+    var container1 = document.createElement('div');
+    var container2 = document.createElement('div');
+
+    let called = false;
+    class Foo extends React.Component {
+      componentDidMount() {
+        called = true;
+        ReactDOM.render(<div>Hello</div>, container2);
+        expect(container2.textContent).toEqual('Hello');
+        ReactDOM.unmountComponentAtNode(container2);
+        expect(container2.textContent).toEqual('');
+      }
+      render() {
+        return <div>{this.props.step}</div>;
+      }
+    }
+
+    ReactDOM.render(<Foo />, container1);
+    expect(called).toEqual(true);
+  });
 });