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)             // Optimized wrapper implementation
  20   Lock() : lock_() {}
  21   ~Lock() {}
  22   void Acquire() { lock_.Lock(); }
  23   void Release() { lock_.Unlock(); }
  24
  25   // If the lock is not held, take it and return true. If the lock is already
  26   // held by another thread, immediately return false. This must not be called
  27   // by a thread already holding the lock (what happens is undefined and an
  28   // assertion may fail).
  29   bool Try() { return lock_.Try(); }
  30
  31   // Null implementation if not debug.
  32   void AssertAcquired() const {}
  33 #else
  34   Lock();
  35   ~Lock();
  36
  37   // NOTE: Although windows critical sections support recursive locks, we do not
  38   // allow this, and we will commonly fire a DCHECK() if a thread attempts to
  39   // acquire the lock a second time (while already holding it).
  40   void Acquire() {
  41     lock_.Lock();
  42     CheckUnheldAndMark();
  43   }
  44   void Release() {
  45     CheckHeldAndUnmark();
  46     lock_.Unlock();
  47   }
  48
  49   bool Try() {
  50     bool rv = lock_.Try();
  51     if (rv) {
  52       CheckUnheldAndMark();
  53     }
  54     return rv;
  55   }
  56
  57   void AssertAcquired() const;
  58 #endif                          // NDEBUG
  59
  60 #if defined(OS_POSIX)
  61   // The posix implementation of ConditionVariable needs to be able
  62   // to see our lock and tweak our debugging counters, as it releases
  63   // and acquires locks inside of pthread_cond_{timed,}wait.
  64   friend class ConditionVariable;
  65 #elif defined(OS_WIN)
  66   // The Windows Vista implementation of ConditionVariable needs the
  67   // native handle of the critical section.
  68   friend class WinVistaCondVar;
  69 #endif
  70
  71  private:
  72 #if !defined(NDEBUG)
  73   // Members and routines taking care of locks assertions.
  74   // Note that this checks for recursive locks and allows them
  75   // if the variable is set.  This is allowed by the underlying implementation
  76   // on windows but not on Posix, so we're doing unneeded checks on Posix.
  77   // It's worth it to share the code.
  78   void CheckHeldAndUnmark();
  79   void CheckUnheldAndMark();
  80
  81   // All private data is implicitly protected by lock_.
  82   // Be VERY careful to only access members under that lock.
  83
  84   // Determines validity of owning_thread_id_.  Needed as we don't have
  85   // a null owning_thread_id_ value.
  86   bool owned_by_thread_;
  87   base::PlatformThreadId owning_thread_id_;
  88 #endif  // NDEBUG
  89
  90   // Platform specific underlying lock implementation.
  91   internal::LockImpl lock_;
  92
  93   DISALLOW_COPY_AND_ASSIGN(Lock);
  94 };
  95
  96 // A helper class that acquires the given Lock while the AutoLock is in scope.
  97 class AutoLock {
  98  public:
  99   struct AlreadyAcquired {};
 100
 101   explicit AutoLock(Lock& lock) : lock_(lock) {
 102     lock_.Acquire();
 103   }
 104
 105   AutoLock(Lock& lock, const AlreadyAcquired&) : lock_(lock) {
 106     lock_.AssertAcquired();
 107   }
 108
 109   ~AutoLock() {
 110     lock_.AssertAcquired();
 111     lock_.Release();
 112   }
 113
 114  private:
 115   Lock& lock_;
 116   DISALLOW_COPY_AND_ASSIGN(AutoLock);
 117 };
 118
 119 // AutoUnlock is a helper that will Release() the |lock| argument in the
 120 // constructor, and re-Acquire() it in the destructor.
 121 class AutoUnlock {
 122  public:
 123   explicit AutoUnlock(Lock& lock) : lock_(lock) {
 124     // We require our caller to have the lock.
 125     lock_.AssertAcquired();
 126     lock_.Release();
 127   }
 128
 129   ~AutoUnlock() {
 130     lock_.Acquire();
 131   }
 132
 133  private:
 134   Lock& lock_;
 135   DISALLOW_COPY_AND_ASSIGN(AutoUnlock);
 136 };
 137
 138 }  // namespace base
 139
 140 #endif  // BASE_SYNCHRONIZATION_LOCK_H_