TCMalloc: reduce waiting time in SpinLockDelay on Linux

third_party/tcmalloc/chromium/src/base/spinlock_linux-inl.h

 /* Copyright (c) 2009, Google Inc.
  * All rights reserved.
  * 
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  * 
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 69 matching lines @@
 
 namespace base {
 namespace internal {
 
 void SpinLockDelay(volatile Atomic32 *w, int32 value, int loop) {
   if (loop != 0) {
     int save_errno = errno;
     struct timespec tm;
     tm.tv_sec = 0;
     if (have_futex) {
+      // Wait between 0-16ms.
       tm.tv_nsec = base::internal::SuggestedDelayNS(loop);
-    } else {
-      tm.tv_nsec = 2000001;   // above 2ms so linux 2.4 doesn't spin
-    }
-    if (have_futex) {
-      tm.tv_nsec *= 16;  // increase the delay; we expect explicit wakeups
+      // Note: since Unlock() is optimized to not do a compare-and-swap,
+      // we can't expect explicit wake-ups. Therefore we shouldn't wait too
+      // long here.
       syscall(__NR_futex, reinterpret_cast<int *>(const_cast<Atomic32 *>(w)),
                 FUTEX_WAIT | futex_private_flag,
                 value, reinterpret_cast<struct kernel_timespec *>(&tm));
     } else {
+      tm.tv_nsec = 2000001;   // above 2ms so linux 2.4 doesn't spin
       nanosleep(&tm, NULL);
     }
     errno = save_errno;
   }
 }
 
 void SpinLockWake(volatile Atomic32 *w, bool all) {
   if (have_futex) {
     syscall(__NR_futex, reinterpret_cast<int *>(const_cast<Atomic32 *>(w)),
               FUTEX_WAKE | futex_private_flag, 1, 0);
   }
 }
 
 } // namespace internal
 } // namespace base