From 2cd213ac3b5d524acfcae85f689e7b3c1df47d2c Mon Sep 17 00:00:00 2001
From: Jeff Genovy <29107334+jefgen@users.noreply.github.com>
Date: Wed, 3 Jul 2019 14:04:36 -0700
Subject: [PATCH] ICU-20689 Fix race condition in UMutex code, leading to
 random crashes.

---
 icu4c/source/common/umutex.cpp | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/icu4c/source/common/umutex.cpp b/icu4c/source/common/umutex.cpp
index d31753ff31..8be43bf960 100644
--- a/icu4c/source/common/umutex.cpp
+++ b/icu4c/source/common/umutex.cpp
@@ -90,7 +90,8 @@ std::mutex *UMutex::getMutex() {
     if (retPtr == nullptr) {
         std::call_once(*pInitFlag, umtx_init);
         std::lock_guard<std::mutex> guard(*initMutex);
-        if (fMutex.load() == nullptr) {
+        retPtr = fMutex.load(std::memory_order_acquire);
+        if (retPtr == nullptr) {
             fMutex = new(fStorage) std::mutex();
             retPtr = fMutex;
             fListLink = gListHead;