Index: lldb/trunk/include/lldb/Utility/Log.h
===================================================================
--- lldb/trunk/include/lldb/Utility/Log.h
+++ lldb/trunk/include/lldb/Utility/Log.h
@@ -96,6 +96,9 @@
     }
   };
 
+
+  static void Initialize();
+
   //------------------------------------------------------------------
   // Static accessors for logging channels
   //------------------------------------------------------------------
@@ -193,6 +196,8 @@
   static uint32_t GetFlags(llvm::raw_ostream &stream, const ChannelMap::value_type &entry,
                            llvm::ArrayRef<const char *> categories);
 
+  static void DisableLoggingChild();
+
   Log(const Log &) = delete;
   void operator=(const Log &) = delete;
 };
Index: lldb/trunk/include/lldb/Utility/Logging.h
===================================================================
--- lldb/trunk/include/lldb/Utility/Logging.h
+++ lldb/trunk/include/lldb/Utility/Logging.h
@@ -62,7 +62,7 @@
 
 Log *GetLogIfAnyCategoriesSet(uint32_t mask);
 
-void InitializeLog();
+void InitializeLldbChannel();
 
 } // namespace lldb_private
 
Index: lldb/trunk/source/Host/posix/ProcessLauncherPosixFork.cpp
===================================================================
--- lldb/trunk/source/Host/posix/ProcessLauncherPosixFork.cpp
+++ lldb/trunk/source/Host/posix/ProcessLauncherPosixFork.cpp
@@ -95,10 +95,6 @@
 
 static void LLVM_ATTRIBUTE_NORETURN ChildFunc(int error_fd,
                                               const ProcessLaunchInfo &info) {
-  // First, make sure we disable all logging. If we are logging to stdout, our
-  // logs can be mistaken for inferior output.
-  Log::DisableAllLogChannels();
-
   // Do not inherit setgid powers.
   if (setgid(getgid()) != 0)
     ExitWithError(error_fd, "setgid");
Index: lldb/trunk/source/Initialization/SystemInitializerCommon.cpp
===================================================================
--- lldb/trunk/source/Initialization/SystemInitializerCommon.cpp
+++ lldb/trunk/source/Initialization/SystemInitializerCommon.cpp
@@ -70,7 +70,7 @@
 #endif
 
   llvm::EnablePrettyStackTrace();
-  InitializeLog();
+  Log::Initialize();
   HostInfo::Initialize();
   static Timer::Category func_cat(LLVM_PRETTY_FUNCTION);
   Timer scoped_timer(func_cat, LLVM_PRETTY_FUNCTION);
Index: lldb/trunk/source/Utility/Log.cpp
===================================================================
--- lldb/trunk/source/Utility/Log.cpp
+++ lldb/trunk/source/Utility/Log.cpp
@@ -32,6 +32,7 @@
 #include <process.h> // for getpid
 #else
 #include <unistd.h>
+#include <pthread.h>
 #endif
 
 using namespace lldb_private;
@@ -181,6 +182,13 @@
   Printf("warning: %s", Content.c_str());
 }
 
+void Log::Initialize() {
+#ifdef LLVM_ON_UNIX
+  pthread_atfork(nullptr, nullptr, &Log::DisableLoggingChild);
+#endif
+  InitializeLldbChannel();
+}
+
 void Log::Register(llvm::StringRef name, Channel &channel) {
   auto iter = g_channel_map->try_emplace(name, channel);
   assert(iter.second == true);
@@ -321,3 +329,11 @@
   message << payload << "\n";
   WriteMessage(message.str());
 }
+
+void Log::DisableLoggingChild() {
+  // Disable logging by clearing out the atomic variable after forking -- if we
+  // forked while another thread held the channel mutex, we would deadlock when
+  // trying to write to the log.
+  for (auto &c: *g_channel_map)
+    c.second.m_channel.log_ptr.store(nullptr, std::memory_order_relaxed);
+}
Index: lldb/trunk/source/Utility/Logging.cpp
===================================================================
--- lldb/trunk/source/Utility/Logging.cpp
+++ lldb/trunk/source/Utility/Logging.cpp
@@ -51,7 +51,7 @@
 
 static Log::Channel g_log_channel(g_categories, LIBLLDB_LOG_DEFAULT);
 
-void lldb_private::InitializeLog() {
+void lldb_private::InitializeLldbChannel() {
   Log::Register("lldb", g_log_channel);
 }