need child specific release that doesn't trigger cv

src/ae.cpp

@@ -859,6 +859,11 @@ void aeReleaseForkLock()
     g_forkLock.downgradeWrite();
 }
 
+void aeReleaseForkLockChild()
+{
+    g_forkLock.downgradeWriteChild();
+}
+
 int aeThreadOwnsLock()
 {
     return fOwnLockOverride || g_lock.fOwnLock();

src/ae.h

@@ -171,6 +171,7 @@ int aeTryAcquireLock(int fWeak);
 void aeThreadOffline();
 void aeReleaseLock();
 void aeReleaseForkLock();
+void aeReleaseForkLockChild();
 int aeThreadOwnsLock();
 void aeSetThreadOwnsLockOverride(int fOverride);
 int aeLockContested(int threshold);

src/readwritelock.h

@@ -77,11 +77,25 @@ public:
         m_cv.notify_all();
     }
 
+    void releaseWriteChild(bool exclusive = true) {
+        std::unique_lock<fastlock> rm(m_readLock);
+        serverAssert(m_writeCount > 0);
+        if (exclusive)
+            m_writeLock.unlock();
+        m_writeCount--;
+        m_writeWaiting = false;
+    }
+
     void downgradeWrite(bool exclusive = true) {
         releaseWrite(exclusive);
         acquireRead();
     }
 
+    void downgradeWriteChild(bool exclusive = true) {
+        releaseWriteChild(exclusive);
+        acquireRead();
+    }
+
     bool hasReader() {
         return m_readCount > 0;
     }

src/server.cpp

@@ -6888,6 +6888,7 @@ int redisFork(int purpose) {
     latencyAddSampleIfNeeded("fork-lock",(ustime()-startWriteLock)/1000);
     if ((childpid = fork()) == 0) {
         /* Child */
+        aeReleaseForkLockChild();
         g_pserver->in_fork_child = purpose;
         setOOMScoreAdj(CONFIG_OOM_BGCHILD);
         setupChildSignalHandlers();