Improve SharedMemory::Lock on Posix and reenable StatsTableTest.MultipleThreads

base/shared_memory.h

 // Copyright (c) 2012 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_SHARED_MEMORY_H_
 #define BASE_SHARED_MEMORY_H_
 #pragma once
 
 #include "build/build_config.h"
 
+#include <string>
+
 #if defined(OS_POSIX)
 #include <stdio.h>
 #include <sys/types.h>
 #include <semaphore.h>
-#include "base/file_descriptor_posix.h"
 #endif
-#include <string>
 
 #include "base/base_export.h"
 #include "base/basictypes.h"
 #include "base/process.h"
 
+#if defined(OS_POSIX)
+#include "base/file_descriptor_posix.h"
+#endif
+
 class FilePath;
 
 namespace base {
 
 // SharedMemoryHandle is a platform specific type which represents
 // the underlying OS handle to a shared memory segment.
 #if defined(OS_WIN)
 typedef HANDLE SharedMemoryHandle;
 typedef HANDLE SharedMemoryLock;
 #elif defined(OS_POSIX)
@@ skipping 161 matching lines @@
   //   Close();
   //   return ok;
   // Note that the memory is unmapped by calling this method, regardless of the
   // return value.
   bool GiveToProcess(ProcessHandle process,
                      SharedMemoryHandle* new_handle) {
     return ShareToProcessCommon(process, new_handle, true);
   }
 
   // Locks the shared memory.
-  // This is a cross-process lock which may be recursively
-  // locked by the same thread.
-  // TODO(port):
-  // WARNING: on POSIX the lock only works across processes, not
-  // across threads.  2 threads in the same process can both grab the
-  // lock at the same time.  There are several solutions for this
-  // (futex, lockf+anon_semaphore) but none are both clean and common
-  // across Mac and Linux.
+  //
+  // WARNING: on POSIX the memory locking primitive only works across
+  // processes, not across threads.  The Lock method is not currently
+  // used in inner loops, so we protect against multiple threads in a
+  // critical section using a class global lock.
   void Lock();
 
 #if defined(OS_WIN)
   // A Lock() implementation with a timeout that also allows setting
   // security attributes on the mutex. sec_attr may be NULL.
   // Returns true if the Lock() has been acquired, false if the timeout was
   // reached.
   bool Lock(uint32 timeout_ms, SECURITY_ATTRIBUTES* sec_attr);
 #endif
 
@@ skipping 42 matching lines @@
   }
 
  private:
   SharedMemory* shared_memory_;
   DISALLOW_COPY_AND_ASSIGN(SharedMemoryAutoLock);
 };
 
 }  // namespace base
 
 #endif  // BASE_SHARED_MEMORY_H_