Index: lib/sanitizer_common/sanitizer_common.h
===================================================================
--- lib/sanitizer_common/sanitizer_common.h
+++ lib/sanitizer_common/sanitizer_common.h
@@ -723,8 +723,10 @@
 void WriteToSyslog(const char *buffer);
 
 #if SANITIZER_MAC
+void DarwinLogInit(bool IsLogThreadsafe);
 void LogFullErrorReport(const char *buffer);
 #else
+INLINE void DarwinLogInit(bool IsLogThreadsafe) {}
 INLINE void LogFullErrorReport(const char *buffer) {}
 #endif
 
Index: lib/sanitizer_common/sanitizer_mac.cc
===================================================================
--- lib/sanitizer_common/sanitizer_mac.cc
+++ lib/sanitizer_common/sanitizer_mac.cc
@@ -392,6 +392,29 @@
   return (signum == SIGSEGV || signum == SIGBUS) && common_flags()->handle_segv;
 }
 
+#ifndef SANITIZER_GO
+uint32_t is_log_threadsafe = 0;
+#endif
+
+// The logging on OS X may call pthread_create so we need the threading
+// environment to be fully initialized.
+//
+// NOTE: It isn't safe to write to the syslog while holding a thread registry
+// lock. If the reporting thread holds the thread registry mutex, and asl_log
+// waits for GCD to dispatch a new thread, the process will deadlock, because
+// the pthread_create wrapper needs to acquire the lock as well. Only set
+// IsLogThreadsafe to true if this deadlock scenario has been ruled out.
+//
+// If multiple sanitizers are enabled, it's safe to set IsLogThreadsafe=true
+// in one runtime and not in the others. That's because no single thread will
+// double-lock a thread-registry lock while preparing a report.
+void DarwinLogInit(bool IsLogThreadsafe) {
+#ifndef SANITIZER_GO
+  atomic_store(reinterpret_cast<atomic_uint32_t *>(&is_log_threadsafe),
+               uint32_t(IsLogThreadsafe), memory_order_release);
+#endif
+}
+
 MacosVersion cached_macos_version = MACOS_VERSION_UNINITIALIZED;
 
 MacosVersion GetMacosVersionInternal() {
@@ -471,10 +494,31 @@
 #endif
 }
 
+void WriteToSyslogIfPossible(const char *str) {
+#ifndef SANITIZER_GO
+  // We can't write to syslog if it's disabled.
+  if (!common_flags()->log_to_syslog)
+    return;
+
+  // We also can't do the write if there's a danger of double-locking a thread
+  // registry lock. See this discussion: https://reviews.llvm.org/D24781
+  if (!atomic_load(reinterpret_cast<atomic_uint32_t *>(&is_log_threadsafe),
+                   memory_order_acquire))
+    return;
+
+  BlockingMutexLock l(&syslog_lock);
+  WriteToSyslog(str);
+#endif
+}
+
 void LogMessageOnPrintf(const char *str) {
-  // Log all printf output to CrashLog.
-  if (common_flags()->abort_on_error)
+  // If the program is about to crash, *just* copy the report into CrashLog.
+  if (common_flags()->abort_on_error) {
     CRAppendCrashLogMessage(str);
+    return;
+  }
+
+  WriteToSyslogIfPossible(str);
 }
 
 void LogFullErrorReport(const char *buffer) {
@@ -501,15 +545,7 @@
 #endif
 
   // Log to syslog.
-  // The logging on OS X may call pthread_create so we need the threading
-  // environment to be fully initialized. Also, this should never be called when
-  // holding the thread registry lock since that may result in a deadlock. If
-  // the reporting thread holds the thread registry mutex, and asl_log waits
-  // for GCD to dispatch a new thread, the process will deadlock, because the
-  // pthread_create wrapper needs to acquire the lock as well.
-  BlockingMutexLock l(&syslog_lock);
-  if (common_flags()->log_to_syslog)
-    WriteToSyslog(buffer);
+  WriteToSyslogIfPossible(buffer);
 
   // The report is added to CrashLog as part of logging all of Printf output.
 #endif
Index: lib/ubsan/ubsan_init.cc
===================================================================
--- lib/ubsan/ubsan_init.cc
+++ lib/ubsan/ubsan_init.cc
@@ -40,6 +40,9 @@
   CacheBinaryName();
   __sanitizer_set_report_path(common_flags()->log_path);
   AndroidLogInit();
+  // Set up logging on Darwin. There is no thread-registry lock in ubsan, so
+  // the logging routine may safely create a thread whenever it needs to.
+  DarwinLogInit(/*IsLogThreadsafe=*/true);
   InitializeCoverage(common_flags()->coverage, common_flags()->coverage_dir);
   CommonInit();
   ubsan_mode = UBSAN_MODE_STANDALONE;
Index: test/ubsan/TestCases/Misc/log-path_test.cc
===================================================================
--- test/ubsan/TestCases/Misc/log-path_test.cc
+++ test/ubsan/TestCases/Misc/log-path_test.cc
@@ -20,6 +20,10 @@
 // RUN: %env_ubsan_opts=log_path='"%t.log"'  %run %t 4
 // RUN: not cat %t.log.*
 
+// We should print out a report even if we're logging to syslog.
+// RUN: %env_ubsan_opts=log_to_syslog=1 %run %t -4 2> %t.out
+// RUN: FileCheck %s --check-prefix=CHECK-ERROR < %t.out
+
 // FIXME: log_path is not supported on Windows yet.
 // XFAIL: win32