Use ptr-to-const in Futex

[folly.git] / folly / detail / Futex.h

diff --git a/folly/detail/Futex.h b/folly/detail/Futex.h
index fe61d0e66417081fcc45c5a569b3b492acbf0306..a2e35020dc2dbafc0981626dff6666ac4a0221b2 100644 (file)
--- a/folly/detail/Futex.h
+++ b/folly/detail/Futex.h
@@ -1,5 +1,5 @@
 /*
- * Copyright 2014 Facebook, Inc.
+ * Copyright 2013-present Facebook, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -17,19 +17,21 @@
 #pragma once
 
 #include <atomic>
+#include <cassert>
 #include <chrono>
 #include <limits>
-#include <assert.h>
-#include <unistd.h>
+
 #include <boost/noncopyable.hpp>
 
+#include <folly/portability/Unistd.h>
+
 namespace folly { namespace detail {
 
 enum class FutexResult {
   VALUE_CHANGED, /* Futex value didn't match expected */
   AWOKEN,        /* futex wait matched with a futex wake */
   INTERRUPTED,   /* Spurious wake-up or signal caused futex wait failure */
-  TIMEDOUT
+  TIMEDOUT,
 };
 
 /**
@@ -44,7 +46,7 @@ enum class FutexResult {
 template <template <typename> class Atom = std::atomic>
 struct Futex : Atom<uint32_t>, boost::noncopyable {
 
-  explicit Futex(uint32_t init = 0) : Atom<uint32_t>(init) {}
+  explicit constexpr Futex(uint32_t init = 0) : Atom<uint32_t>(init) {}
 
   /** Puts the thread to sleep if this->load() == expected.  Returns true when
    *  it is returning because it has consumed a wake() event, false for any
@@ -80,18 +82,36 @@ struct Futex : Atom<uint32_t>, boost::noncopyable {
         "futexWaitUntil only knows std::chrono::{system_clock,steady_clock}");
     assert((std::is_same<Clock, system_clock>::value) || Clock::is_steady);
 
-    auto duration = absTime.time_since_epoch();
+    // We launder the clock type via a std::chrono::duration so that we
+    // can compile both the true and false branch.  Tricky case is when
+    // steady_clock has a higher precision than system_clock (Xcode 6,
+    // for example), for which time_point<system_clock> construction
+    // refuses to do an implicit duration conversion.  (duration is
+    // happy to implicitly convert its denominator causing overflow, but
+    // refuses conversion that might cause truncation.)  We use explicit
+    // duration_cast to work around this.  Truncation does not actually
+    // occur (unless Duration != Clock::duration) because the missing
+    // implicit conversion is in the untaken branch.
+    Duration absTimeDuration = absTime.time_since_epoch();
     if (std::is_same<Clock, system_clock>::value) {
-      time_point<system_clock> absSystemTime(duration);
+      time_point<system_clock> absSystemTime(
+          duration_cast<system_clock::duration>(absTimeDuration));
       return futexWaitImpl(expected, &absSystemTime, nullptr, waitMask);
     } else {
-      time_point<steady_clock> absSteadyTime(duration);
+      time_point<steady_clock> absSteadyTime(
+          duration_cast<steady_clock::duration>(absTimeDuration));
       return futexWaitImpl(expected, nullptr, &absSteadyTime, waitMask);
     }
   }
 
-  /** Wakens up to count waiters where (waitMask & wakeMask) != 0,
-   *  returning the number of awoken threads. */
+  /** Wakens up to count waiters where (waitMask & wakeMask) !=
+   *  0, returning the number of awoken threads, or -1 if an error
+   *  occurred.  Note that when constructing a concurrency primitive
+   *  that can guard its own destruction, it is likely that you will
+   *  want to ignore EINVAL here (as well as making sure that you
+   *  never touch the object after performing the memory store that
+   *  is the linearization point for unlock or control handoff).
+   *  See https://sourceware.org/bugzilla/show_bug.cgi?id=13690 */
   int futexWake(int count = std::numeric_limits<int>::max(),
                 uint32_t wakeMask = -1);
 
@@ -105,8 +125,8 @@ struct Futex : Atom<uint32_t>, boost::noncopyable {
    *  is the same as system_clock on some platforms. */
   FutexResult futexWaitImpl(
       uint32_t expected,
-      std::chrono::time_point<std::chrono::system_clock>* absSystemTime,
-      std::chrono::time_point<std::chrono::steady_clock>* absSteadyTime,
+      std::chrono::system_clock::time_point const* absSystemTime,
+      std::chrono::steady_clock::time_point const* absSteadyTime,
       uint32_t waitMask);
 };
 
@@ -118,29 +138,31 @@ struct EmulatedFutexAtomic : public std::atomic<T> {
   EmulatedFutexAtomic() noexcept = default;
   constexpr /* implicit */ EmulatedFutexAtomic(T init) noexcept
       : std::atomic<T>(init) {}
-  EmulatedFutexAtomic(const EmulatedFutexAtomic& rhs) = delete;
+  // It doesn't copy or move
+  EmulatedFutexAtomic(EmulatedFutexAtomic&& rhs) = delete;
 };
 
 /* Available specializations, with definitions elsewhere */
 
-template<>
+template <>
 int Futex<std::atomic>::futexWake(int count, uint32_t wakeMask);
 
-template<>
+template <>
 FutexResult Futex<std::atomic>::futexWaitImpl(
-      uint32_t expected,
-      std::chrono::time_point<std::chrono::system_clock>* absSystemTime,
-      std::chrono::time_point<std::chrono::steady_clock>* absSteadyTime,
-      uint32_t waitMask);
+    uint32_t expected,
+    std::chrono::system_clock::time_point const* absSystemTime,
+    std::chrono::steady_clock::time_point const* absSteadyTime,
+    uint32_t waitMask);
 
-template<>
+template <>
 int Futex<EmulatedFutexAtomic>::futexWake(int count, uint32_t wakeMask);
 
-template<>
+template <>
 FutexResult Futex<EmulatedFutexAtomic>::futexWaitImpl(
-      uint32_t expected,
-      std::chrono::time_point<std::chrono::system_clock>* absSystemTime,
-      std::chrono::time_point<std::chrono::steady_clock>* absSteadyTime,
-      uint32_t waitMask);
+    uint32_t expected,
+    std::chrono::system_clock::time_point const* absSystemTime,
+    std::chrono::steady_clock::time_point const* absSteadyTime,
+    uint32_t waitMask);
 
-}}
+} // namespace detail
+} // namespace folly