diff --git a/Source/WebKit/ChangeLog b/Source/WebKit/ChangeLog
index 3d6fb9b..c22b5bf 100644
--- a/Source/WebKit/ChangeLog
+++ b/Source/WebKit/ChangeLog
@@ -1,3 +1,35 @@
+2019-10-22  Chris Dumez  <cdumez@apple.com>
+
+        Simplify "Unexpectedly Resumed" assertion handling
+        https://bugs.webkit.org/show_bug.cgi?id=203254
+
+        Reviewed by Geoffrey Garen.
+
+        When the WebContent process gets resumed from suspension, it now unconditionally takes a
+        process assertion on behalf on the UIProcess and sends a ProcessDidResume IPC to the
+        UIProcess. The UIProcess then sends a DidHandleProcessWasResumed IPC back after handing
+        the ProcessDidResume IPC allowing the WebContent process to release its assertion on
+        behalf on the UIProcess.
+
+        The previous code was racy because it relied on the m_processIsSuspended flag, which was
+        queried and set from different threads. Also, the 'unexpectedly resumed' naming was
+        confusing since we'd often take this assertion whenever the WebProcess got resumed,
+        wether unexpected or not, simply because the processTaskStateDidChange IPC won the race
+        with the ProcessDidResume IPC from the UIProcess.
+
+        * UIProcess/Cocoa/WebProcessProxyCocoa.mm:
+        (WebKit::WebProcessProxy::processWasResumed):
+        (WebKit::WebProcessProxy::processWasUnexpectedlyUnsuspended): Deleted.
+        * UIProcess/WebProcessProxy.h:
+        * UIProcess/WebProcessProxy.messages.in:
+        * WebProcess/WebProcess.cpp:
+        (WebKit::WebProcess::processDidResume):
+        * WebProcess/WebProcess.h:
+        * WebProcess/WebProcess.messages.in:
+        * WebProcess/cocoa/WebProcessCocoa.mm:
+        (WebKit::WebProcess::processTaskStateDidChange):
+        (WebKit::WebProcess::didHandleProcessWasResumed):
+
 2019-10-22  youenn fablet  <youenn@apple.com>
 
         Remove mayHaveServiceWorkerRegisteredForOrigin
diff --git a/Source/WebKit/UIProcess/Cocoa/WebProcessProxyCocoa.mm b/Source/WebKit/UIProcess/Cocoa/WebProcessProxyCocoa.mm
index 88aa1ce..83435f9 100644
--- a/Source/WebKit/UIProcess/Cocoa/WebProcessProxyCocoa.mm
+++ b/Source/WebKit/UIProcess/Cocoa/WebProcessProxyCocoa.mm
@@ -37,6 +37,7 @@
 #import "WebProcessPool.h"
 #import <sys/sysctl.h>
 #import <wtf/NeverDestroyed.h>
+#import <wtf/Scope.h>
 #import <wtf/spi/darwin/SandboxSPI.h>
 
 namespace WebKit {
@@ -190,19 +191,22 @@
 #endif
 
 #if PLATFORM(IOS_FAMILY)
-void WebProcessProxy::processWasUnexpectedlyUnsuspended()
+void WebProcessProxy::processWasResumed()
 {
+    auto exitScope = makeScopeExit([this] {
+        send(Messages::WebProcess::ParentProcessDidHandleProcessWasResumed(), 0);
+    });
+
     if (m_throttler.shouldBeRunnable()) {
-        // The process becoming unsuspended was not unexpected; it likely was notified of its running state
-        // before receiving a processDidResume() message from the UIProcess.
+        // The process becoming unsuspended was not unexpected.
         return;
     }
 
     // The WebProcess was awakened by something other than the UIProcess. Take out an assertion for a
     // limited duration to allow whatever task needs to be accomplished time to complete.
-    RELEASE_LOG(ProcessSuspension, "%p - WebProcessProxy::processWasUnexpectedlyUnsuspended()", this);
+    RELEASE_LOG(ProcessSuspension, "%p - WebProcessProxy::processWasResumed() Process was unexpectedly resumed, starting background activity", this);
     auto backgroundActivityTimeoutHandler = [activityToken = m_throttler.backgroundActivityToken(), weakThis = makeWeakPtr(this)] {
-        RELEASE_LOG(ProcessSuspension, "%p - WebProcessProxy::processWasUnexpectedlyUnsuspended() - lambda, background activity timed out", weakThis.get());
+        RELEASE_LOG(ProcessSuspension, "%p - WebProcessProxy::processWasResumed() - lambda, background activity timed out", weakThis.get());
     };
     m_unexpectedActivityTimer = makeUnique<WebCore::DeferrableOneShotTimer>(WTFMove(backgroundActivityTimeoutHandler), unexpectedActivityDuration);
 }
diff --git a/Source/WebKit/UIProcess/WebProcessProxy.h b/Source/WebKit/UIProcess/WebProcessProxy.h
index 8a47e813ff..967cdb7 100644
--- a/Source/WebKit/UIProcess/WebProcessProxy.h
+++ b/Source/WebKit/UIProcess/WebProcessProxy.h
@@ -326,7 +326,7 @@
 #endif
 
 #if PLATFORM(IOS_FAMILY)
-    void processWasUnexpectedlyUnsuspended();
+    void processWasResumed();
 #endif
 
     void webPageMediaStateDidChange(WebPageProxy&);
diff --git a/Source/WebKit/UIProcess/WebProcessProxy.messages.in b/Source/WebKit/UIProcess/WebProcessProxy.messages.in
index e119e6c..3a183ad 100644
--- a/Source/WebKit/UIProcess/WebProcessProxy.messages.in
+++ b/Source/WebKit/UIProcess/WebProcessProxy.messages.in
@@ -71,7 +71,7 @@
 #endif
 
 #if PLATFORM(IOS_FAMILY)
-    ProcessWasUnexpectedlyUnsuspended()
+    ProcessWasResumed()
 #endif
 
     # Plug-in messages.
diff --git a/Source/WebKit/WebProcess/WebProcess.cpp b/Source/WebKit/WebProcess/WebProcess.cpp
index b87e750..96ebc08 100644
--- a/Source/WebKit/WebProcess/WebProcess.cpp
+++ b/Source/WebKit/WebProcess/WebProcess.cpp
@@ -1539,13 +1539,6 @@
     m_webSQLiteDatabaseTracker.setIsSuspended(false);
     SQLiteDatabase::setIsDatabaseOpeningForbidden(false);
     accessibilityProcessSuspendedNotification(false);
-    {
-        LockHolder holder(m_unexpectedlyResumedUIAssertionLock);
-        if (m_unexpectedlyResumedUIAssertion) {
-            RELEASE_LOG(ProcessSuspension, "%p - WebProcess::processDidResume() Releasing 'Unexpectedly resumed' assertion", this);
-            m_unexpectedlyResumedUIAssertion = nullptr;
-        }
-    }
 #endif
 
 #if ENABLE(VIDEO)
diff --git a/Source/WebKit/WebProcess/WebProcess.h b/Source/WebKit/WebProcess/WebProcess.h
index a68ab63..8be7260 100644
--- a/Source/WebKit/WebProcess/WebProcess.h
+++ b/Source/WebKit/WebProcess/WebProcess.h
@@ -455,6 +455,7 @@
 
 #if PLATFORM(IOS_FAMILY)
     void processTaskStateDidChange(ProcessTaskStateObserver::TaskState) final;
+    void parentProcessDidHandleProcessWasResumed();
     bool shouldFreezeOnSuspension() const;
     void updateFreezerStatus();
 #endif
@@ -535,8 +536,8 @@
 #if PLATFORM(IOS_FAMILY)
     WebSQLiteDatabaseTracker m_webSQLiteDatabaseTracker;
     Ref<ProcessTaskStateObserver> m_taskStateObserver;
-    Lock m_unexpectedlyResumedUIAssertionLock;
-    RetainPtr<BKSProcessAssertion> m_unexpectedlyResumedUIAssertion;
+    Lock m_processWasResumedUIAssertionLock;
+    RetainPtr<BKSProcessAssertion> m_processWasResumedUIAssertion;
 #endif
 
     enum PageMarkingLayersAsVolatileCounterType { };
diff --git a/Source/WebKit/WebProcess/WebProcess.messages.in b/Source/WebKit/WebProcess/WebProcess.messages.in
index c7f3e0a..d88d6075 100644
--- a/Source/WebKit/WebProcess/WebProcess.messages.in
+++ b/Source/WebKit/WebProcess/WebProcess.messages.in
@@ -156,6 +156,7 @@
 
 #if PLATFORM(IOS_FAMILY)
     UnblockAccessibilityServer(WebKit::SandboxExtension::Handle handle)
+    ParentProcessDidHandleProcessWasResumed();
 #endif
 
 #if PLATFORM(GTK) || PLATFORM(WPE)
diff --git a/Source/WebKit/WebProcess/cocoa/WebProcessCocoa.mm b/Source/WebKit/WebProcess/cocoa/WebProcessCocoa.mm
index 51bc8a5..ca6f4e2 100644
--- a/Source/WebKit/WebProcess/cocoa/WebProcessCocoa.mm
+++ b/Source/WebKit/WebProcess/cocoa/WebProcessCocoa.mm
@@ -297,43 +297,41 @@
 {
     // NOTE: This will be called from a background thread.
     RELEASE_LOG(ProcessSuspension, "%p - WebProcess::processTaskStateDidChange() - taskState(%d)", this, taskState);
-    if (taskState == ProcessTaskStateObserver::None)
+    if (taskState != ProcessTaskStateObserver::Running)
         return;
 
-    if (taskState == ProcessTaskStateObserver::Suspended) {
-        if (m_processIsSuspended)
-            return;
-
-        RELEASE_LOG(ProcessSuspension, "%p - WebProcess::processTaskStateChanged() - unexpectedly entered Suspended state", this);
-        return;
-    }
-
-    if (!m_processIsSuspended)
-        return;
-
-    LockHolder holder(m_unexpectedlyResumedUIAssertionLock);
-    if (m_unexpectedlyResumedUIAssertion)
+    LockHolder holder(m_processWasResumedUIAssertionLock);
+    if (m_processWasResumedUIAssertion)
         return;
 
     // We were awakened from suspension unexpectedly. Notify the WebProcessProxy, but take a process assertion on our parent PID
     // to ensure that it too is awakened.
-    RELEASE_LOG(ProcessSuspension, "%p - WebProcess::processTaskStateChanged() Taking 'Unexpectedly resumed' assertion", this);
-    m_unexpectedlyResumedUIAssertion = adoptNS([[BKSProcessAssertion alloc] initWithPID:parentProcessConnection()->remoteProcessID() flags:BKSProcessAssertionPreventTaskSuspend reason:BKSProcessAssertionReasonFinishTask name:@"Unexpectedly resumed" withHandler:nil]);
+    RELEASE_LOG(ProcessSuspension, "%p - WebProcess::processTaskStateChanged() Taking 'WebProcess was resumed' assertion on behalf on UIProcess", this);
+    m_processWasResumedUIAssertion = adoptNS([[BKSProcessAssertion alloc] initWithPID:parentProcessConnection()->remoteProcessID() flags:BKSProcessAssertionPreventTaskSuspend reason:BKSProcessAssertionReasonFinishTask name:@"WebProcess was resumed" withHandler:nil]);
 
-    auto releaseAssertion = [this] {
-        LockHolder holder(m_unexpectedlyResumedUIAssertionLock);
-        if (!m_unexpectedlyResumedUIAssertion)
-            return;
-
-        RELEASE_LOG(ProcessSuspension, "%p - WebProcess::processTaskStateChanged() Releasing 'Unexpectedly resumed' assertion due to time out", this);
-        [m_unexpectedlyResumedUIAssertion invalidate];
-        m_unexpectedlyResumedUIAssertion = nullptr;
+    m_processWasResumedUIAssertion.get().invalidationHandler = [this] {
+        LockHolder holder(m_processWasResumedUIAssertionLock);
+        RELEASE_LOG(ProcessSuspension, "%p - WebProcess::processTaskStateChanged() Releasing 'WebProcess was resumed' assertion on behalf on UIProcess due invalidation", this);
+        [m_processWasResumedUIAssertion invalidate];
+        m_processWasResumedUIAssertion = nullptr;
     };
 
-    m_unexpectedlyResumedUIAssertion.get().invalidationHandler = releaseAssertion;
-    parentProcessConnection()->send(Messages::WebProcessProxy::ProcessWasUnexpectedlyUnsuspended(), 0);
-    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(5 * NSEC_PER_SEC)), dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), releaseAssertion);
+    // This will cause the parent process to send a ParentProcessDidHandleProcessWasResumed IPC back, so that we can release our assertion on its behalf.
+    // We are not using sendWithAsyncReply() because it would not be thread-safe.
+    parentProcessConnection()->send(Messages::WebProcessProxy::ProcessWasResumed(), 0);
 }
+
+void WebProcess::parentProcessDidHandleProcessWasResumed()
+{
+    LockHolder holder(m_processWasResumedUIAssertionLock);
+    ASSERT(m_processWasResumedUIAssertion);
+
+    RELEASE_LOG(ProcessSuspension, "%p - WebProcess::parentProcessDidHandleProcessWasResumed() Releasing 'WebProcess was resumed' assertion on behalf on UIProcess", this);
+
+    [m_processWasResumedUIAssertion invalidate];
+    m_processWasResumedUIAssertion = nullptr;
+}
+
 #endif
 
 #if PLATFORM(IOS_FAMILY)