ceph/src/rocksdb/third-party/folly/folly/synchronization/detail/Spin.h

   1 //  Copyright (c) 2011-present, Facebook, Inc.  All rights reserved.
   2 //  This source code is licensed under both the GPLv2 (found in the
   3 //  COPYING file in the root directory) and Apache 2.0 License
   4 //  (found in the LICENSE.Apache file in the root directory).
   5
   6 #pragma once
   7
   8 #include <algorithm>
   9 #include <chrono>
  10 #include <thread>
  11
  12 #include <folly/portability/Asm.h>
  13 #include <folly/synchronization/WaitOptions.h>
  14
  15 namespace folly {
  16 namespace detail {
  17
  18 enum class spin_result {
  19   success, // condition passed
  20   timeout, // exceeded deadline
  21   advance, // exceeded current wait-options component timeout
  22 };
  23
  24 template <typename Clock, typename Duration, typename F>
  25 spin_result spin_pause_until(
  26     std::chrono::time_point<Clock, Duration> const& deadline,
  27     WaitOptions const& opt,
  28     F f) {
  29   if (opt.spin_max() <= opt.spin_max().zero()) {
  30     return spin_result::advance;
  31   }
  32
  33   auto tbegin = Clock::now();
  34   while (true) {
  35     if (f()) {
  36       return spin_result::success;
  37     }
  38
  39     auto const tnow = Clock::now();
  40     if (tnow >= deadline) {
  41       return spin_result::timeout;
  42     }
  43
  44     //  Backward time discontinuity in Clock? revise pre_block starting point
  45     tbegin = std::min(tbegin, tnow);
  46     if (tnow >= tbegin + opt.spin_max()) {
  47       return spin_result::advance;
  48     }
  49
  50     //  The pause instruction is the polite way to spin, but it doesn't
  51     //  actually affect correctness to omit it if we don't have it. Pausing
  52     //  donates the full capabilities of the current core to its other
  53     //  hyperthreads for a dozen cycles or so.
  54     asm_volatile_pause();
  55   }
  56 }
  57
  58 template <typename Clock, typename Duration, typename F>
  59 spin_result spin_yield_until(
  60     std::chrono::time_point<Clock, Duration> const& deadline,
  61     F f) {
  62   while (true) {
  63     if (f()) {
  64       return spin_result::success;
  65     }
  66
  67     auto const max = std::chrono::time_point<Clock, Duration>::max();
  68     if (deadline != max && Clock::now() >= deadline) {
  69       return spin_result::timeout;
  70     }
  71
  72     std::this_thread::yield();
  73   }
  74 }
  75
  76 } // namespace detail
  77 } // namespace folly