diff --git a/compiler-rt/lib/profile/InstrProfilingUtil.c b/compiler-rt/lib/profile/InstrProfilingUtil.c
--- a/compiler-rt/lib/profile/InstrProfilingUtil.c
+++ b/compiler-rt/lib/profile/InstrProfilingUtil.c
@@ -207,8 +207,9 @@
   f = fdopen(fd, "r+b");
 #elif defined(_WIN32)
   // FIXME: Use the wide variants to handle Unicode filenames.
-  HANDLE h = CreateFileA(ProfileName, GENERIC_READ | GENERIC_WRITE, 0, 0,
-                         OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL, 0);
+  HANDLE h = CreateFileA(ProfileName, GENERIC_READ | GENERIC_WRITE,
+                         FILE_SHARE_READ | FILE_SHARE_WRITE, 0, OPEN_ALWAYS,
+                         FILE_ATTRIBUTE_NORMAL, 0);
   if (h == INVALID_HANDLE_VALUE)
     return NULL;
 
@@ -218,6 +219,10 @@
     return NULL;
   }
 
+  if (lprofLockFd(fd) != 0)
+    PROF_WARN("Data may be corrupted during profile merging : %s\n",
+              "Fail to obtain file lock due to system limit.");
+
   f = _fdopen(fd, "r+b");
   if (f == 0) {
     CloseHandle(h);
diff --git a/compiler-rt/test/profile/Windows/Inputs/instrprof-multiprocess.c b/compiler-rt/test/profile/Windows/Inputs/instrprof-multiprocess.c
new file mode 100644
--- /dev/null
+++ b/compiler-rt/test/profile/Windows/Inputs/instrprof-multiprocess.c
@@ -0,0 +1,89 @@
+/* This is a test case where the parent process forks 10 children
+ * which contend to merge profile data to the same file. With
+ * file locking support, the data from each child should not
+ * be lost.
+ */
+#include <stdio.h>
+#include <stdlib.h>
+#include <windows.h>
+
+void spawn_child(PROCESS_INFORMATION *pi, int child_num) {
+  wchar_t child_str[10];
+  _itow(child_num, child_str, 10);
+  if (!SetEnvironmentVariableW(L"CHILD_NUM", child_str)) {
+    printf("SetEnvironmentVariableW failed (0x%8lx).\n", GetLastError());
+    fflush(stdout);
+    exit(1);
+  }
+
+  STARTUPINFOW si;
+  memset(&si, 0, sizeof(si));
+  si.cb = sizeof(si);
+
+  memset(pi, 0, sizeof(PROCESS_INFORMATION));
+
+  if (!CreateProcessW(NULL,              // No module name (use command line)
+                      GetCommandLineW(), // Command line
+                      NULL,              // Process handle not inheritable
+                      NULL,              // Thread handle not inheritable
+                      TRUE,              // Set handle inheritance to TRUE
+                      0,                 // No flags
+                      NULL,              // Use parent's environment block
+                      NULL,              // Use parent's starting directory
+                      &si, pi)) {
+    printf("CreateProcess failed (0x%08lx).\n", GetLastError());
+    fflush(stdout);
+    exit(1);
+  }
+}
+
+int wait_child(PROCESS_INFORMATION *pi) {
+  WaitForSingleObject(pi->hProcess, INFINITE);
+
+  DWORD exit_code;
+  if (!GetExitCodeProcess(pi->hProcess, &exit_code)) {
+    printf("GetExitCodeProcess failed (0x%08lx).\n", GetLastError());
+    fflush(stdout);
+    exit(1);
+  }
+
+  CloseHandle(pi->hProcess);
+  CloseHandle(pi->hThread);
+
+  return exit_code;
+}
+
+#define NUM_CHILDREN 10
+
+int foo(int num) {
+  if (num < (NUM_CHILDREN / 2)) {
+    return 1;
+  } else if (num < NUM_CHILDREN) {
+    return 2;
+  }
+  return 3;
+}
+
+int main(int argc, char *argv[]) {
+  char *child_str = getenv("CHILD_NUM");
+  if (!child_str) {
+    PROCESS_INFORMATION child[NUM_CHILDREN];
+    // In parent
+    for (int i = 0; i < NUM_CHILDREN; i++) {
+      spawn_child(&child[i], i);
+    }
+    for (int i = 0; i < NUM_CHILDREN; i++) {
+      wait_child(&child[i]);
+    }
+    return 0;
+  } else {
+    // In child
+    int child_num = atoi(child_str);
+    int result = foo(child_num);
+    if (result == 3) {
+      fprintf(stderr, "Invalid child count!");
+      return 1;
+    }
+    return 0;
+  }
+}
diff --git a/compiler-rt/test/profile/Windows/instrprof-multiprocess.test b/compiler-rt/test/profile/Windows/instrprof-multiprocess.test
new file mode 100644
--- /dev/null
+++ b/compiler-rt/test/profile/Windows/instrprof-multiprocess.test
@@ -0,0 +1,10 @@
+RUN: %clang_profgen %S/Inputs/instrprof-multiprocess.c -o %t
+RUN: rm -f %t_*.profraw
+RUN: env LLVM_PROFILE_FILE=%t_%m.profraw %run %t
+RUN: llvm-profdata show --counts -function=foo %t_*.profraw | FileCheck %s
+
+CHECK: Counters:
+CHECK:   foo:
+CHECK:     Function count: 10
+CHECK:     Block counts: [5, 5]
+CHECK: Functions shown: 1
diff --git a/compiler-rt/test/profile/Windows/lit.local.cfg.py b/compiler-rt/test/profile/Windows/lit.local.cfg.py
new file mode 100644
--- /dev/null
+++ b/compiler-rt/test/profile/Windows/lit.local.cfg.py
@@ -0,0 +1,9 @@
+def getRoot(config):
+  if not config.parent:
+    return config
+  return getRoot(config.parent)
+
+root = getRoot(config)
+
+if root.host_os not in ['Windows']:
+  config.unsupported = True