Index: lib/profile/GCDAProfiling.c
===================================================================
--- lib/profile/GCDAProfiling.c
+++ lib/profile/GCDAProfiling.c
@@ -341,7 +341,8 @@
 COMPILER_RT_VISIBILITY
 void llvm_gcda_start_file(const char *orig_filename, const char version[4],
                           uint32_t checksum) {
-  const char *mode = "r+b";
+  const char *mode;
+  int errnum = 0;
   filename = mangle_filename(orig_filename);
 
   /* Try just opening the file. */
@@ -349,29 +350,51 @@
   fd = open(filename, O_RDWR | O_BINARY);
 
   if (fd == -1) {
-    /* Try opening the file, creating it if necessary. */
-    new_file = 1;
-    mode = "w+b";
-    fd = open(filename, O_RDWR | O_CREAT | O_BINARY, 0644);
+    /* Try opening the file, creating it if necessary.
+       To avoid a race condition, we use O_EXCL. */
+    fd = open(filename, O_RDWR | O_CREAT | O_EXCL | O_BINARY, 0644);
     if (fd == -1) {
-      /* Try creating the directories first then opening the file. */
-      __llvm_profile_recursive_mkdir(filename);
-      fd = open(filename, O_RDWR | O_CREAT | O_BINARY, 0644);
-      if (fd == -1) {
-        /* Bah! It's hopeless. */
-        int errnum = errno;
-        fprintf(stderr, "profiling: %s: cannot open: %s\n", filename,
-                strerror(errnum));
-        return;
+      errnum = errno;
+      if (errnum == EEXIST) {
+        /* In the meantime, an other process has created the file. */
+        fd = open(filename, O_RDWR | O_BINARY);
+        errnum = errno;
+      } else if (errnum == ENOENT) {
+        /* Try creating the directories first then opening the file. */
+        __llvm_profile_recursive_mkdir(filename);
+        fd = open(filename, O_RDWR | O_CREAT | O_EXCL | O_BINARY, 0644);
+        if (fd == -1) {
+          errnum = errno;
+          if (errnum == EEXIST) {
+            fd = open(filename, O_RDWR | O_BINARY);
+            errnum = errno;
+          }
+        } else {
+          new_file = 1;
+        }
       }
+    } else {
+      new_file = 1;
     }
   }
 
+  if (fd == -1) {
+    /* Bah! It's hopeless. */
+    fprintf(stderr, "profiling: %s: cannot open: %s\n", filename,
+            strerror(errnum));
+    return;
+  }
+
   /* Try to flock the file to serialize concurrent processes writing out to the
    * same GCDA. This can fail if the filesystem doesn't support it, but in that
    * case we'll just carry on with the old racy behaviour and hope for the best.
    */
-  lprofLockFd(fd);
+  if (lprofLockFd(fd) != 0) {
+    errnum = errno;
+    fprintf(stderr, "profiling: %s: cannot lock: %s\n", filename,
+            strerror(errnum));
+  }
+  mode = new_file == 1 ? "w+b" : "r+b";
   output_file = fdopen(fd, mode);
 
   /* Initialize the write buffer. */
Index: lib/profile/InstrProfilingUtil.c
===================================================================
--- lib/profile/InstrProfilingUtil.c
+++ lib/profile/InstrProfilingUtil.c
@@ -125,8 +125,7 @@
   }
   return 0;
 #else
-  flock(fd, LOCK_EX);
-  return 0;
+  return flock(fd, LOCK_EX);
 #endif
 }
 
@@ -150,8 +149,7 @@
   }
   return 0;
 #else
-  flock(fd, LOCK_UN);
-  return 0;
+  return flock(fd, LOCK_UN);
 #endif
 }
 
Index: test/profile/Inputs/instrprof-gcov-prefix.c
===================================================================
--- /dev/null
+++ test/profile/Inputs/instrprof-gcov-prefix.c
@@ -0,0 +1,3 @@
+int main() {
+    return 0;
+}
Index: test/profile/Posix/instrprof-gcov-prefix.test
===================================================================
--- /dev/null
+++ test/profile/Posix/instrprof-gcov-prefix.test
@@ -0,0 +1,11 @@
+RUN: mkdir -p %t.d
+RUN: cd %t.d
+
+RUN: %clang --coverage -o %t %S/../Inputs/instrprof-gcov-prefix.c
+RUN: test -f instrprof-gcov-prefix.gcno
+
+RUN: rm -fr %t.d/dir1
+RUN: GCOV_PREFIX=%t.d/dir1/dir2/dir3 %run %t
+RUN: GCOV_PREFIX=%t.d/dir1/dir2/dir3 %run %t
+RUN: test -f %t.d/dir1/dir2/dir3/`pwd`/instrprof-gcov-prefix.gcda
+RUN: rm -fr %t.d/dir1