diff --git a/compiler-rt/lib/profile/InstrProfilingFile.c b/compiler-rt/lib/profile/InstrProfilingFile.c
--- a/compiler-rt/lib/profile/InstrProfilingFile.c
+++ b/compiler-rt/lib/profile/InstrProfilingFile.c
@@ -424,13 +424,10 @@
  * its instrumented shared libraries dump profile data into their own data file.
 */
 static FILE *openFileForMerging(const char *ProfileFileName, int *MergeDone) {
-  FILE *ProfileFile = NULL;
+  FILE *ProfileFile = getProfileFile();
   int rc;
 
-  ProfileFile = getProfileFile();
-  if (ProfileFile) {
-    lprofLockFileHandle(ProfileFile);
-  } else {
+  if (!ProfileFile) {
     createProfileDir(ProfileFileName);
     ProfileFile = lprofOpenFileEx(ProfileFileName);
   }
@@ -481,9 +478,6 @@
 
   if (OutputFile == getProfileFile()) {
     fflush(OutputFile);
-    if (doMerging()) {
-      lprofUnlockFileHandle(OutputFile);
-    }
   } else {
     fclose(OutputFile);
   }
diff --git a/compiler-rt/test/profile/ContinuousSyncMode/online-merging-windows.c b/compiler-rt/test/profile/ContinuousSyncMode/online-merging-windows.c
new file mode 100644
--- /dev/null
+++ b/compiler-rt/test/profile/ContinuousSyncMode/online-merging-windows.c
@@ -0,0 +1,156 @@
+// REQUIRES: windows
+
+// Test the online merging mode (%m) along with continuous mode (%c).
+//
+// Split files & cd into a temporary directory.
+// RUN: rm -rf %t.dir && split-file %s %t.dir && cd %t.dir
+//
+// Create two DLLs and a driver program that uses them.
+// RUN: %clang_pgogen foo.c -mllvm -instrprof-atomic-counter-update-all=1 -mllvm -runtime-counter-relocation=true -fuse-ld=lld -Wl,-dll -o %t.dir/foo.dll
+// RUN: %clang_pgogen bar.c -mllvm -instrprof-atomic-counter-update-all=1 -mllvm -runtime-counter-relocation=true -fuse-ld=lld -Wl,-dll -o %t.dir/bar.dll
+// RUN: %clang_pgogen main.c -o main.exe %t.dir/foo.lib %t.dir/bar.lib -mllvm -instrprof-atomic-counter-update-all=1 -mllvm -runtime-counter-relocation=true -fuse-ld=lld
+//
+// === Round 1 ===
+// Test merging+continuous mode without any file contention.
+//
+// RUN: env LLVM_PROFILE_FILE="%t.dir/profdir/%m%c.profraw" %run %t.dir/main.exe nospawn
+// RUN: llvm-profdata merge -o %t.profdata %t.dir/profdir
+// RUN: llvm-profdata show --counts --all-functions %t.profdata | FileCheck %s -check-prefix=ROUND1
+
+// ROUND1-LABEL: Counters:
+// ROUND1-DAG:   foo:
+// ROUND1-DAG:     Hash: 0x{{.*}}
+// ROUND1-DAG:     Counters: 1
+// ROUND1-DAG:     Block counts: [1]
+// ROUND1-DAG:   bar:
+// ROUND1-DAG:     Hash: 0x{{.*}}
+// ROUND1-DAG:     Counters: 1
+// ROUND1-DAG:     Block counts: [1]
+// ROUND1-DAG:   main:
+// ROUND1-DAG:     Hash: 0x{{.*}}
+// ROUND1-LABEL: Instrumentation level: IR
+//
+// === Round 2 ===
+// Test merging+continuous mode with some file contention.
+//
+// RUN: env LLVM_PROFILE_FILE="%t.dir/profdir/%m%c.profraw" %run %t.dir/main.exe spawn
+// RUN: llvm-profdata merge -o %t.profdata %t.dir/profdir
+// RUN: llvm-profdata show --counts --all-functions %t.profdata | FileCheck %s -check-prefix=ROUND2
+
+// ROUND2-LABEL: Counters:
+// ROUND2-DAG:   foo:
+// ROUND2-DAG:     Hash: 0x{{.*}}
+// ROUND2-DAG:     Counters: 1
+// ROUND2-DAG:     Block counts: [97]
+// ROUND2-DAG:   bar:
+// ROUND2-DAG:     Hash: 0x{{.*}}
+// ROUND2-DAG:     Counters: 1
+// ROUND2-DAG:     Block counts: [97]
+// ROUND2-DAG:   main:
+// ROUND2-DAG:     Hash: 0x{{.*}}
+// ROUND2-LABEL: Instrumentation level: IR
+
+//--- foo.c
+__declspec(dllexport) void foo(void) {}
+
+//--- bar.c
+__declspec(dllexport) void bar(void) {}
+
+//--- main.c
+#include <stdio.h>
+#include <string.h>
+#include <windows.h>
+
+
+const int num_child_procs_to_spawn = 32;
+
+extern int __llvm_profile_is_continuous_mode_enabled(void);
+extern char *__llvm_profile_get_filename(void);
+
+__declspec(dllimport) void foo(void);
+__declspec(dllimport) void bar(void);
+
+// Change to "#define" for debug output.
+#undef DEBUG_TEST
+
+#ifdef DEBUG_TEST
+#  define DEBUG(...) fprintf(stderr, __VA_ARGS__);
+#else
+#  define DEBUG(...)
+#endif
+
+int main(int argc, char *const argv[]) {
+  if (argc < 2) {
+    DEBUG("Requires at least one argument.\n");
+    return 1;
+  }
+  if (strcmp(argv[1], "nospawn") == 0) {
+    DEBUG(
+        "Hello from child (pid = %lu, cont-mode-enabled = %d, profile = %s).\n",
+        GetCurrentProcessId(), __llvm_profile_is_continuous_mode_enabled(),
+        __llvm_profile_get_filename());
+
+    foo();
+    bar();
+    return 0;
+  } else if (strcmp(argv[1], "spawn") == 0) {
+    // This is the start of Round 2.
+    // Expect Counts[dsoX] = 1, as this was the state at the end of Round 1.
+    int I;
+    HANDLE child_pids[num_child_procs_to_spawn];
+    for (I = 0; I < num_child_procs_to_spawn; ++I) {
+      foo(); // Counts[dsoX] += 2 * num_child_procs_to_spawn
+      bar();
+
+      DEBUG("Spawning child with argv = {%s, %s, NULL} and envp = {%s, NULL}\n",
+            child_argv[0], child_argv[1], child_envp[0]);
+
+      // Start the child process.
+      STARTUPINFO si;
+      ZeroMemory(&si, sizeof(si));
+      PROCESS_INFORMATION pi;
+      ZeroMemory(&pi, sizeof(pi));
+      if (!CreateProcess(NULL,               // No module name (use command line)
+                         "main.exe nospawn", // Command line
+                         NULL,               // Process handle not inheritable
+                         NULL,               // Thread handle not inheritable
+                         FALSE,              // Set handle inheritance to FALSE
+                         0,                  // No creation flags
+                         NULL,               // Use parent's environment block
+                         NULL,               // Use parent's starting directory
+                         &si,                // Pointer to STARTUPINFO structure
+                         &pi)                // Pointer to PROCESS_INFORMATION structure
+      ) {
+        fprintf(stderr, "Child %d could not be spawned: %lu\n", I,
+                GetLastError());
+        return 1;
+      }
+      child_pids[I] = pi.hProcess;
+
+      DEBUG("Spawned child %d (pid = %zu).\n", I, pi.dwProcessId);
+    }
+    for (I = 0; I < num_child_procs_to_spawn; ++I) {
+      foo(); // Counts[dsoX] += num_child_procs_to_spawn
+      bar();
+
+      DWORD exit_code;
+      WaitForSingleObject(child_pids[I], INFINITE);
+      if (!GetExitCodeProcess(child_pids[I], &exit_code)) {
+        fprintf(stderr, "Failed to get exit code of child %d.\n", I);
+        return 1;
+      }
+      if (exit_code != 0) {
+        fprintf(stderr, "Child %d did not exit with code 0.\n", I);
+        return 1;
+      }
+    }
+
+    // At the end of Round 2, we have:
+    // Counts[dsoX] = 1 + (2 * num_child_procs_to_spawn) + num_child_procs_to_spawn
+    //              = 97
+
+    return 0;
+  }
+
+  return 1;
+}
diff --git a/compiler-rt/test/profile/ContinuousSyncMode/runtime-counter-relocation.c b/compiler-rt/test/profile/ContinuousSyncMode/runtime-counter-relocation.c
--- a/compiler-rt/test/profile/ContinuousSyncMode/runtime-counter-relocation.c
+++ b/compiler-rt/test/profile/ContinuousSyncMode/runtime-counter-relocation.c
@@ -1,4 +1,4 @@
-// REQUIRES: linux
+// REQUIRES: linux || windows
 
 // RUN: %clang -fprofile-instr-generate -fcoverage-mapping -mllvm -runtime-counter-relocation=true -o %t.exe %s
 // RUN: echo "garbage" > %t.profraw