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lock.h (2901B)

      1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
      2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
      3 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
      4 // Use of this source code is governed by a BSD-style license that can be
      5 // found in the LICENSE file.
      6 
      7 #ifndef BASE_LOCK_H_
      8 #define BASE_LOCK_H_
      9 
     10 #include "base/basictypes.h"
     11 #include "base/lock_impl.h"
     12 #include "base/platform_thread.h"
     13 
     14 // A convenient wrapper for an OS specific critical section.
     15 class Lock {
     16 public:
     17  // Optimized wrapper implementation
     18  Lock() = default;
     19  ~Lock() = default;
     20  void Acquire() { lock_.Lock(); }
     21  void Release() { lock_.Unlock(); }
     22 
     23  // If the lock is not held, take it and return true. If the lock is already
     24  // held by another thread, immediately return false. This must not be called
     25  // by a thread already holding the lock (what happens is undefined and an
     26  // assertion may fail).
     27  bool Try() { return lock_.Try(); }
     28 
     29  // Null implementation if not debug.
     30  void AssertAcquired() const {}
     31 
     32  // Whether Lock mitigates priority inversion when used from different thread
     33  // priorities.
     34  static bool HandlesMultipleThreadPriorities() {
     35 #if defined(XP_UNIX)
     36    // POSIX mitigates priority inversion by setting the priority of a thread
     37    // holding a Lock to the maximum priority of any other thread waiting on it.
     38    return base::internal::LockImpl::PriorityInheritanceAvailable();
     39 #else
     40    // Windows mitigates priority inversion by randomly boosting the priority of
     41    // ready threads.
     42    // https://msdn.microsoft.com/library/windows/desktop/ms684831.aspx
     43    return true;
     44 #endif
     45  }
     46 
     47  // Both Windows and POSIX implementations of ConditionVariable need to be
     48  // able to see our lock and tweak our debugging counters, as they release and
     49  // acquire locks inside of their condition variable APIs.
     50  friend class ConditionVariable;
     51 
     52 private:
     53  // Platform specific underlying lock implementation.
     54  ::base::internal::LockImpl lock_;
     55 
     56  DISALLOW_COPY_AND_ASSIGN(Lock);
     57 };
     58 
     59 // A helper class that acquires the given Lock while the AutoLock is in scope.
     60 class AutoLock {
     61 public:
     62  struct AlreadyAcquired {};
     63 
     64  explicit AutoLock(Lock& lock) : lock_(lock) { lock_.Acquire(); }
     65 
     66  AutoLock(Lock& lock, const AlreadyAcquired&) : lock_(lock) {
     67    lock_.AssertAcquired();
     68  }
     69 
     70  ~AutoLock() {
     71    lock_.AssertAcquired();
     72    lock_.Release();
     73  }
     74 
     75 private:
     76  Lock& lock_;
     77  DISALLOW_COPY_AND_ASSIGN(AutoLock);
     78 };
     79 
     80 // AutoUnlock is a helper that will Release() the |lock| argument in the
     81 // constructor, and re-Acquire() it in the destructor.
     82 class AutoUnlock {
     83 public:
     84  explicit AutoUnlock(Lock& lock) : lock_(lock) {
     85    // We require our caller to have the lock.
     86    lock_.AssertAcquired();
     87    lock_.Release();
     88  }
     89 
     90  ~AutoUnlock() { lock_.Acquire(); }
     91 
     92 private:
     93  Lock& lock_;
     94  DISALLOW_COPY_AND_ASSIGN(AutoUnlock);
     95 };
     96 
     97 #endif  // BASE_LOCK_H_