Index: compiler-rt/trunk/lib/scudo/standalone/fuchsia.cc
===================================================================
--- compiler-rt/trunk/lib/scudo/standalone/fuchsia.cc
+++ compiler-rt/trunk/lib/scudo/standalone/fuchsia.cc
@@ -14,8 +14,10 @@
 #include "mutex.h"
 #include "string_utils.h"
 
-#include <limits.h> // for PAGE_SIZE
-#include <stdlib.h> // for getenv()
+#include <lib/sync/mutex.h> // for sync_mutex_t
+#include <limits.h>         // for PAGE_SIZE
+#include <stdlib.h>         // for getenv()
+#include <zircon/compiler.h>
 #include <zircon/sanitizer.h>
 #include <zircon/syscalls.h>
 
@@ -90,7 +92,8 @@
   }
 
   uintptr_t P;
-  zx_vm_option_t MapFlags = ZX_VM_PERM_READ | ZX_VM_PERM_WRITE;
+  zx_vm_option_t MapFlags =
+      ZX_VM_PERM_READ | ZX_VM_PERM_WRITE | ZX_VM_ALLOW_FAULTS;
   const uint64_t Offset =
       Addr ? reinterpret_cast<uintptr_t>(Addr) - Data->VmarBase : 0;
   if (Offset)
@@ -149,18 +152,21 @@
 
 const char *getEnv(const char *Name) { return getenv(Name); }
 
-void BlockingMutex::wait() {
-  const zx_status_t Status =
-      _zx_futex_wait(reinterpret_cast<zx_futex_t *>(OpaqueStorage), MtxSleeping,
-                     ZX_HANDLE_INVALID, ZX_TIME_INFINITE);
-  if (Status != ZX_ERR_BAD_STATE)
-    CHECK_EQ(Status, ZX_OK); // Normal race
+// Note: we need to flag these methods with __TA_NO_THREAD_SAFETY_ANALYSIS
+// because the Fuchsia implementation of sync_mutex_t has clang thread safety
+// annotations. Were we to apply proper capability annotations to the top level
+// BlockingMutex class itself, they would not be needed. As it stands, the
+// thread analysis thinks that we are locking the mutex and accidentally leaving
+// it locked on the way out.
+void BlockingMutex::lock() __TA_NO_THREAD_SAFETY_ANALYSIS {
+  // Size and alignment must be compatible between both types.
+  COMPILER_CHECK(sizeof(sync_mutex_t) <= sizeof(OpaqueStorage));
+  COMPILER_CHECK(!(alignof(decltype(OpaqueStorage)) % alignof(sync_mutex_t)));
+  sync_mutex_lock(reinterpret_cast<sync_mutex_t *>(OpaqueStorage));
 }
 
-void BlockingMutex::wake() {
-  const zx_status_t Status =
-      _zx_futex_wake(reinterpret_cast<zx_futex_t *>(OpaqueStorage), 1);
-  CHECK_EQ(Status, ZX_OK);
+void BlockingMutex::unlock() __TA_NO_THREAD_SAFETY_ANALYSIS {
+  sync_mutex_unlock(reinterpret_cast<sync_mutex_t *>(OpaqueStorage));
 }
 
 u64 getMonotonicTime() { return _zx_clock_get_monotonic(); }
Index: compiler-rt/trunk/lib/scudo/standalone/linux.cc
===================================================================
--- compiler-rt/trunk/lib/scudo/standalone/linux.cc
+++ compiler-rt/trunk/lib/scudo/standalone/linux.cc
@@ -84,14 +84,22 @@
 // Calling getenv should be fine (c)(tm) at any time.
 const char *getEnv(const char *Name) { return getenv(Name); }
 
-void BlockingMutex::wait() {
-  syscall(SYS_futex, reinterpret_cast<uptr>(OpaqueStorage), FUTEX_WAIT_PRIVATE,
-          MtxSleeping, nullptr, nullptr, 0);
-}
-
-void BlockingMutex::wake() {
-  syscall(SYS_futex, reinterpret_cast<uptr>(OpaqueStorage), FUTEX_WAKE_PRIVATE,
-          1, nullptr, nullptr, 0);
+void BlockingMutex::lock() {
+  atomic_u32 *M = reinterpret_cast<atomic_u32 *>(&OpaqueStorage);
+  if (atomic_exchange(M, MtxLocked, memory_order_acquire) == MtxUnlocked)
+    return;
+  while (atomic_exchange(M, MtxSleeping, memory_order_acquire) != MtxUnlocked)
+    syscall(SYS_futex, reinterpret_cast<uptr>(OpaqueStorage),
+            FUTEX_WAIT_PRIVATE, MtxSleeping, nullptr, nullptr, 0);
+}
+
+void BlockingMutex::unlock() {
+  atomic_u32 *M = reinterpret_cast<atomic_u32 *>(&OpaqueStorage);
+  const u32 V = atomic_exchange(M, MtxUnlocked, memory_order_release);
+  DCHECK_NE(V, MtxUnlocked);
+  if (V == MtxSleeping)
+    syscall(SYS_futex, reinterpret_cast<uptr>(OpaqueStorage),
+            FUTEX_WAKE_PRIVATE, 1, nullptr, nullptr, 0);
 }
 
 u64 getMonotonicTime() {
Index: compiler-rt/trunk/lib/scudo/standalone/mutex.h
===================================================================
--- compiler-rt/trunk/lib/scudo/standalone/mutex.h
+++ compiler-rt/trunk/lib/scudo/standalone/mutex.h
@@ -57,34 +57,19 @@
   void operator=(const SpinMutex &) = delete;
 };
 
-enum MutexState { MtxUnlocked = 0, MtxLocked = 1, MtxSleeping = 2 };
-
 class BlockingMutex {
 public:
-  explicit constexpr BlockingMutex(LinkerInitialized) : OpaqueStorage{0} {}
+  explicit constexpr BlockingMutex(LinkerInitialized) : OpaqueStorage{} {}
   BlockingMutex() { memset(this, 0, sizeof(*this)); }
-  void wait();
-  void wake();
-  void lock() {
-    atomic_u32 *M = reinterpret_cast<atomic_u32 *>(&OpaqueStorage);
-    if (atomic_exchange(M, MtxLocked, memory_order_acquire) == MtxUnlocked)
-      return;
-    while (atomic_exchange(M, MtxSleeping, memory_order_acquire) != MtxUnlocked)
-      wait();
-  }
-  void unlock() {
-    atomic_u32 *M = reinterpret_cast<atomic_u32 *>(&OpaqueStorage);
-    const u32 V = atomic_exchange(M, MtxUnlocked, memory_order_release);
-    DCHECK_NE(V, MtxUnlocked);
-    if (V == MtxSleeping)
-      wake();
-  }
+  void lock();
+  void unlock();
   void checkLocked() {
     atomic_u32 *M = reinterpret_cast<atomic_u32 *>(&OpaqueStorage);
     CHECK_NE(MtxUnlocked, atomic_load_relaxed(M));
   }
 
 private:
+  enum MutexState { MtxUnlocked = 0, MtxLocked = 1, MtxSleeping = 2 };
   uptr OpaqueStorage[1];
 };