base/synchronization/lock.h

   1 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #ifndef BASE_SYNCHRONIZATION_LOCK_H_
   6 #define BASE_SYNCHRONIZATION_LOCK_H_
   7
   8 #include "base/base_export.h"
   9 #include "base/synchronization/lock_impl.h"
  10 #include "base/threading/platform_thread.h"
  11
  12 namespace base {
  13
  14 // A convenient wrapper for an OS specific critical section.  The only real
  15 // intelligence in this class is in debug mode for the support for the
  16 // AssertAcquired() method.
  17 class BASE_EXPORT Lock {
  18  public:
  19 #if defined(NDEBUG) && !defined(DCHECK_ALWAYS_ON)
  20    // Optimized wrapper implementation
  21   Lock() : lock_() {}
  22   ~Lock() {}
  23   void Acquire() { lock_.Lock(); }
  24   void Release() { lock_.Unlock(); }
  25
  26   // If the lock is not held, take it and return true. If the lock is already
  27   // held by another thread, immediately return false. This must not be called
  28   // by a thread already holding the lock (what happens is undefined and an
  29   // assertion may fail).
  30   bool Try() { return lock_.Try(); }
  31
  32   // Null implementation if not debug.
  33   void AssertAcquired() const {}
  34 #else
  35   Lock();
  36   ~Lock();
  37
  38   // NOTE: Although windows critical sections support recursive locks, we do not
  39   // allow this, and we will commonly fire a DCHECK() if a thread attempts to
  40   // acquire the lock a second time (while already holding it).
  41   void Acquire() {
  42     lock_.Lock();
  43     CheckUnheldAndMark();
  44   }
  45   void Release() {
  46     CheckHeldAndUnmark();
  47     lock_.Unlock();
  48   }
  49
  50   bool Try() {
  51     bool rv = lock_.Try();
  52     if (rv) {
  53       CheckUnheldAndMark();
  54     }
  55     return rv;
  56   }
  57
  58   void AssertAcquired() const;
  59 #endif  // NDEBUG && !DCHECK_ALWAYS_ON
  60
  61 #if defined(OS_POSIX)
  62   // The posix implementation of ConditionVariable needs to be able
  63   // to see our lock and tweak our debugging counters, as it releases
  64   // and acquires locks inside of pthread_cond_{timed,}wait.
  65   friend class ConditionVariable;
  66 #elif defined(OS_WIN)
  67   // The Windows Vista implementation of ConditionVariable needs the
  68   // native handle of the critical section.
  69   friend class WinVistaCondVar;
  70 #endif
  71
  72  private:
  73 #if !defined(NDEBUG) || defined(DCHECK_ALWAYS_ON)
  74   // Members and routines taking care of locks assertions.
  75   // Note that this checks for recursive locks and allows them
  76   // if the variable is set.  This is allowed by the underlying implementation
  77   // on windows but not on Posix, so we're doing unneeded checks on Posix.
  78   // It's worth it to share the code.
  79   void CheckHeldAndUnmark();
  80   void CheckUnheldAndMark();
  81
  82   // All private data is implicitly protected by lock_.
  83   // Be VERY careful to only access members under that lock.
  84   base::PlatformThreadRef owning_thread_ref_;
  85 #endif  // !NDEBUG || DCHECK_ALWAYS_ON
  86
  87   // Platform specific underlying lock implementation.
  88   internal::LockImpl lock_;
  89
  90   DISALLOW_COPY_AND_ASSIGN(Lock);
  91 };
  92
  93 // A helper class that acquires the given Lock while the AutoLock is in scope.
  94 class AutoLock {
  95  public:
  96   struct AlreadyAcquired {};
  97
  98   explicit AutoLock(Lock& lock) : lock_(lock) {
  99     lock_.Acquire();
 100   }
 101
 102   AutoLock(Lock& lock, const AlreadyAcquired&) : lock_(lock) {
 103     lock_.AssertAcquired();
 104   }
 105
 106   ~AutoLock() {
 107     lock_.AssertAcquired();
 108     lock_.Release();
 109   }
 110
 111  private:
 112   Lock& lock_;
 113   DISALLOW_COPY_AND_ASSIGN(AutoLock);
 114 };
 115
 116 // AutoUnlock is a helper that will Release() the |lock| argument in the
 117 // constructor, and re-Acquire() it in the destructor.
 118 class AutoUnlock {
 119  public:
 120   explicit AutoUnlock(Lock& lock) : lock_(lock) {
 121     // We require our caller to have the lock.
 122     lock_.AssertAcquired();
 123     lock_.Release();
 124   }
 125
 126   ~AutoUnlock() {
 127     lock_.Acquire();
 128   }
 129
 130  private:
 131   Lock& lock_;
 132   DISALLOW_COPY_AND_ASSIGN(AutoUnlock);
 133 };
 134
 135 }  // namespace base
 136
 137 #endif  // BASE_SYNCHRONIZATION_LOCK_H_