base/lock.h

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#ifndef BASE_LOCK_H_
#define BASE_LOCK_H_

#include "base/lock_impl.h"

// A convenient wrapper for a critical section.
//
// NOTE: A thread may acquire the same lock multiple times, but it must call
// Release for each call to Acquire in order to finally release the lock.
//
// Complication: UnitTest for DeathTests catch DCHECK exceptions, so we need
// to write code assuming DCHECK will throw.  This means we need to save any
// assertable value in a local until we can safely throw.
class Lock {
 public:
  Lock();
  ~Lock();
  void Acquire();
  void Release();
  // If the lock is not held, take it and return true. If the lock is already
  // held by something else, immediately return false.
  bool Try();

  // Return the underlying lock implementation.
  // TODO(awalker): refactor lock and condition variables so that this is
  // unnecessary.
  LockImpl* lock_impl() { return &lock_; }

 private:
  LockImpl lock_;  // User-supplied underlying lock implementation.

  // All private data is implicitly protected by spin_lock_.
  // Be VERY careful to only access under that lock.
  int32 recursion_count_shadow_;

  // Allow access to GetCurrentThreadRecursionCount()
  friend class AutoUnlock;
  int32 GetCurrentThreadRecursionCount();

#ifndef NDEBUG
  // Even in Debug mode, the expensive tallies won't be calculated by default.
  bool recursion_used_;
  int32 acquisition_count_;

  int32 contention_count_;
#endif  // NDEBUG

  DISALLOW_COPY_AND_ASSIGN(Lock);
};

// A helper class that acquires the given Lock while the AutoLock is in scope.
class AutoLock {
 public:
  AutoLock(Lock& lock) : lock_(lock) {
    lock_.Acquire();
  }

  ~AutoLock() {
    lock_.Release();
  }

 private:
  Lock& lock_;
  DISALLOW_COPY_AND_ASSIGN(AutoLock);
};

// AutoUnlock is a helper class for ConditionVariable instances
// that is analogous to AutoLock.  It provides for nested Releases
// of a lock for the Wait functionality of a ConditionVariable class.
// The destructor automatically does the corresponding Acquire
// calls (to return to the initial nested lock state).

// Instances of AutoUnlock can ***ONLY*** validly be constructed if the
// caller currently holds the lock provided as the constructor's argument.
// If that ***REQUIREMENT*** is violated in debug mode, a DCHECK will
// be generated in the Lock class.  In production (non-debug),
// the results are undefined (and probably bad) if the caller
// is not already holding the indicated lock.
class ConditionVariable;
class AutoUnlock {
 private:  // Everything is private, so only our friend can use us.
  friend class ConditionVariable;  // The only user of this class.
  explicit AutoUnlock(Lock& lock);
  ~AutoUnlock();

  Lock* lock_;
  int release_count_;
};

#endif  // BASE_LOCK_H_