Index: compiler-rt/trunk/lib/asan/asan_posix.cc
===================================================================
--- compiler-rt/trunk/lib/asan/asan_posix.cc
+++ compiler-rt/trunk/lib/asan/asan_posix.cc
@@ -33,6 +33,7 @@
 namespace __asan {
 
 void AsanOnSIGSEGV(int, void *siginfo, void *context) {
+  ScopedDeadlySignal signal_scope(GetCurrentThread());
   uptr addr = (uptr)((siginfo_t*)siginfo)->si_addr;
   int code = (int)((siginfo_t*)siginfo)->si_code;
   // Write the first message using the bullet-proof write.
Index: compiler-rt/trunk/lib/asan/asan_stack.h
===================================================================
--- compiler-rt/trunk/lib/asan/asan_stack.h
+++ compiler-rt/trunk/lib/asan/asan_stack.h
@@ -35,6 +35,10 @@
   stack->size = 0;
   if (LIKELY(asan_inited)) {
     if ((t = GetCurrentThread()) && !t->isUnwinding()) {
+      // On FreeBSD the slow unwinding that leverages _Unwind_Backtrace()
+      // yields the call stack of the signal's handler and not of the code
+      // that raised the signal (as it does on Linux).
+      if (SANITIZER_FREEBSD && t->isInDeadlySignal()) fast = true;
       uptr stack_top = t->stack_top();
       uptr stack_bottom = t->stack_bottom();
       ScopedUnwinding unwind_scope(t);
Index: compiler-rt/trunk/lib/asan/asan_thread.h
===================================================================
--- compiler-rt/trunk/lib/asan/asan_thread.h
+++ compiler-rt/trunk/lib/asan/asan_thread.h
@@ -108,6 +108,10 @@
   bool isUnwinding() const { return unwinding_; }
   void setUnwinding(bool b) { unwinding_ = b; }
 
+  // True if we are in a deadly signal handler.
+  bool isInDeadlySignal() const { return in_deadly_signal_; }
+  void setInDeadlySignal(bool b) { in_deadly_signal_ = b; }
+
   AsanThreadLocalMallocStorage &malloc_storage() { return malloc_storage_; }
   AsanStats &stats() { return stats_; }
 
@@ -133,6 +137,7 @@
   AsanThreadLocalMallocStorage malloc_storage_;
   AsanStats stats_;
   bool unwinding_;
+  bool in_deadly_signal_;
 };
 
 // ScopedUnwinding is a scope for stacktracing member of a context
@@ -147,6 +152,20 @@
   AsanThread *thread;
 };
 
+// ScopedDeadlySignal is a scope for handling deadly signals.
+class ScopedDeadlySignal {
+ public:
+  explicit ScopedDeadlySignal(AsanThread *t) : thread(t) {
+    if (thread) thread->setInDeadlySignal(true);
+  }
+  ~ScopedDeadlySignal() {
+    if (thread) thread->setInDeadlySignal(false);
+  }
+
+ private:
+  AsanThread *thread;
+};
+
 struct CreateThreadContextArgs {
   AsanThread *thread;
   StackTrace *stack;
Index: compiler-rt/trunk/lib/sanitizer_common/sanitizer_stacktrace.h
===================================================================
--- compiler-rt/trunk/lib/sanitizer_common/sanitizer_stacktrace.h
+++ compiler-rt/trunk/lib/sanitizer_common/sanitizer_stacktrace.h
@@ -29,6 +29,15 @@
 # define SANITIZER_CAN_FAST_UNWIND 1
 #endif
 
+// Fast unwind is the only option on Mac for now; we will need to
+// revisit this macro when slow unwind works on Mac, see
+// https://code.google.com/p/address-sanitizer/issues/detail?id=137
+#if SANITIZER_MAC
+# define SANITIZER_CAN_SLOW_UNWIND 0
+#else
+# define SANITIZER_CAN_SLOW_UNWIND 1
+#endif
+
 struct StackTrace {
   const uptr *trace;
   uptr size;
@@ -40,13 +49,9 @@
   void Print() const;
 
   static bool WillUseFastUnwind(bool request_fast_unwind) {
-    // Check if fast unwind is available. Fast unwind is the only option on Mac.
-    // It is also the only option on FreeBSD as the slow unwinding that
-    // leverages _Unwind_Backtrace() yields the call stack of the signal's
-    // handler and not of the code that raised the signal (as it does on Linux).
     if (!SANITIZER_CAN_FAST_UNWIND)
       return false;
-    else if (SANITIZER_MAC != 0 || SANITIZER_FREEBSD != 0)
+    else if (!SANITIZER_CAN_SLOW_UNWIND)
       return true;
     return request_fast_unwind;
   }