Disallow Lock class support for recursive locking

base/lock.h

 // Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_LOCK_H_
 #define BASE_LOCK_H_
 
 #include "base/lock_impl.h"
 
-// A convenient wrapper for a critical section.
+// A convenient wrapper for an OS specific 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.
+// NOTE: Although windows critical sections support recursive locks, we do not
+// allow this, and we will commonly fire a DCHECK() if a thread attempts to
+// acquire the lock a second time (while already holding it).
 //
 // 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.
+  // held by another thread, 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.
+#ifndef NDEBUG
+  // All private data is implicitly protected by lock_.
+  // Be VERY careful to only access members 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_;
+  bool recursion_used_;      // Allow debugging to continued after a DCHECK().
+  int32 acquisition_count_;  // Number of times lock was acquired.
+  int32 contention_count_;   // Number of times there was contention.
 #endif  // NDEBUG
 
   DISALLOW_COPY_AND_ASSIGN(Lock);
 };
 
 // A helper class that acquires the given Lock while the AutoLock is in scope.
 class AutoLock {
  public:
   explicit 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.
+// AutoUnlock is a helper class for ConditionVariable that will Release() the
+// lock argument in the constructor, and re-Acquire() it in the destructor.
 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();
+
+  explicit AutoUnlock(Lock& lock) : lock_(&lock) {
+    // We require our caller to have the lock.
+    lock_->Release();
+  }
+
+  ~AutoUnlock() {
+    lock_->Acquire();
+  }
 
   Lock* lock_;
-  int release_count_;
 };
 
 #endif  // BASE_LOCK_H_