Index: compiler-rt/trunk/lib/sanitizer_common/sanitizer_mac.cc
===================================================================
--- compiler-rt/trunk/lib/sanitizer_common/sanitizer_mac.cc
+++ compiler-rt/trunk/lib/sanitizer_common/sanitizer_mac.cc
@@ -191,7 +191,8 @@
 
 uptr internal_sigprocmask(int how, __sanitizer_sigset_t *set,
                           __sanitizer_sigset_t *oldset) {
-  return sigprocmask(how, set, oldset);
+  // Don't use sigprocmask here, because it affects all threads.
+  return pthread_sigmask(how, set, oldset);
 }
 
 // Doesn't call pthread_atfork() handlers (but not available on 10.6).
Index: compiler-rt/trunk/test/tsan/Darwin/signals-blocked.cc
===================================================================
--- compiler-rt/trunk/test/tsan/Darwin/signals-blocked.cc
+++ compiler-rt/trunk/test/tsan/Darwin/signals-blocked.cc
@@ -0,0 +1,75 @@
+// RUN: %clangxx_tsan %s -o %t && %run %t 2>&1 | FileCheck %s
+
+#include <pthread.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <signal.h>
+#include <sys/types.h>
+#include <sys/time.h>
+#include <unistd.h>
+#include <errno.h>
+
+volatile bool signal_delivered;
+
+static void handler(int sig) {
+  if (sig == SIGALRM)
+    signal_delivered = true;
+}
+
+static void* thr(void *p) {
+  sigset_t sigset;
+  sigemptyset(&sigset);
+  sigaddset(&sigset, SIGALRM);
+  int ret = pthread_sigmask(SIG_UNBLOCK, &sigset, NULL);
+  if (ret) abort();
+
+  struct sigaction act = {};
+  act.sa_handler = &handler;
+  if (sigaction(SIGALRM, &act, 0)) {
+    perror("sigaction");
+    exit(1);
+  }
+
+  itimerval t;
+  t.it_value.tv_sec = 0;
+  t.it_value.tv_usec = 10000;
+  t.it_interval = t.it_value;
+  if (setitimer(ITIMER_REAL, &t, 0)) {
+    perror("setitimer");
+    exit(1);
+  }
+
+  while (!signal_delivered) {
+    usleep(1000);
+  }
+
+  t.it_value.tv_usec = 0;
+  if (setitimer(ITIMER_REAL, &t, 0)) {
+    perror("setitimer");
+    exit(1);
+  }
+
+  fprintf(stderr, "SIGNAL DELIVERED\n");
+
+  return 0;
+}
+
+int main() {
+  sigset_t sigset;
+  sigemptyset(&sigset);
+  sigaddset(&sigset, SIGALRM);
+  int ret = pthread_sigmask(SIG_BLOCK, &sigset, NULL);
+  if (ret) abort();
+
+  pthread_t th;
+  pthread_create(&th, 0, thr, 0);
+  pthread_join(th, 0);
+
+  fprintf(stderr, "DONE\n");
+  return 0;
+}
+
+// CHECK-NOT: WARNING: ThreadSanitizer:
+// CHECK: SIGNAL DELIVERED
+// CHECK: DONE
+// CHECK-NOT: WARNING: ThreadSanitizer: