base/lock.h

   1 // Copyright (c) 2006-2009 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_LOCK_H_
   6 #define BASE_LOCK_H_
   7 #pragma once
   8
   9 #include "base/lock_impl.h"
  10
  11 // A convenient wrapper for an OS specific critical section.  The only real
  12 // intelligence in this class is in debug mode for the support for the
  13 // AssertAcquired() method.
  14
  15 class Lock {
  16  public:
  17 #if defined(NDEBUG)             // Optimized wrapper implementation
  18   Lock() : lock_() {}
  19   ~Lock() {}
  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 #else
  32   Lock();
  33   ~Lock() {}
  34
  35   // NOTE: Although windows critical sections support recursive locks, we do not
  36   // allow this, and we will commonly fire a DCHECK() if a thread attempts to
  37   // acquire the lock a second time (while already holding it).
  38   void Acquire() {
  39     lock_.Lock();
  40     CheckUnheldAndMark();
  41   }
  42   void Release() {
  43     CheckHeldAndUnmark();
  44     lock_.Unlock();
  45   }
  46
  47   bool Try() {
  48     bool rv = lock_.Try();
  49     if (rv) {
  50       CheckUnheldAndMark();
  51     }
  52     return rv;
  53   }
  54
  55   void AssertAcquired() const;
  56 #endif                          // NDEBUG
  57
  58 #if defined(OS_POSIX)
  59   // The posix implementation of ConditionVariable needs to be able
  60   // to see our lock and tweak our debugging counters, as it releases
  61   // and acquires locks inside of pthread_cond_{timed,}wait.
  62   // Windows doesn't need to do this as it calls the Lock::* methods.
  63   friend class ConditionVariable;
  64 #endif
  65
  66  private:
  67 #if !defined(NDEBUG)
  68   // Members and routines taking care of locks assertions.
  69   // Note that this checks for recursive locks and allows them
  70   // if the variable is set.  This is allowed by the underlying implementation
  71   // on windows but not on Posix, so we're doing unneeded checks on Posix.
  72   // It's worth it to share the code.
  73   void CheckHeldAndUnmark();
  74   void CheckUnheldAndMark();
  75
  76   // All private data is implicitly protected by lock_.
  77   // Be VERY careful to only access members under that lock.
  78
  79   // Determines validity of owning_thread_id_.  Needed as we don't have
  80   // a null owning_thread_id_ value.
  81   bool owned_by_thread_;
  82   PlatformThreadId owning_thread_id_;
  83 #endif  // NDEBUG
  84
  85   LockImpl lock_;  // Platform specific underlying lock implementation.
  86
  87   DISALLOW_COPY_AND_ASSIGN(Lock);
  88 };
  89
  90 // A helper class that acquires the given Lock while the AutoLock is in scope.
  91 class AutoLock {
  92  public:
  93   explicit AutoLock(Lock& lock) : lock_(lock) {
  94     lock_.Acquire();
  95   }
  96
  97   ~AutoLock() {
  98     lock_.AssertAcquired();
  99     lock_.Release();
 100   }
 101
 102  private:
 103   Lock& lock_;
 104   DISALLOW_COPY_AND_ASSIGN(AutoLock);
 105 };
 106
 107 // AutoUnlock is a helper that will Release() the |lock| argument in the
 108 // constructor, and re-Acquire() it in the destructor.
 109 class AutoUnlock {
 110  public:
 111   explicit AutoUnlock(Lock& lock) : lock_(lock) {
 112     // We require our caller to have the lock.
 113     lock_.AssertAcquired();
 114     lock_.Release();
 115   }
 116
 117   ~AutoUnlock() {
 118     lock_.Acquire();
 119   }
 120
 121  private:
 122   Lock& lock_;
 123   DISALLOW_COPY_AND_ASSIGN(AutoUnlock);
 124 };
 125
 126 #endif  // BASE_LOCK_H_