src/content/renderer/shared_memory_seqlock_reader.cc

   1 // Copyright 2013 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 #include "shared_memory_seqlock_reader.h"
   6
   7 namespace internal {
   8
   9 SharedMemorySeqLockReaderBase::SharedMemorySeqLockReaderBase() { }
  10
  11 SharedMemorySeqLockReaderBase::~SharedMemorySeqLockReaderBase() { }
  12
  13 void*
  14 SharedMemorySeqLockReaderBase::InitializeSharedMemory(
  15     base::SharedMemoryHandle shared_memory_handle, size_t buffer_size) {
  16   renderer_shared_memory_handle_ = shared_memory_handle;
  17   if (!base::SharedMemory::IsHandleValid(renderer_shared_memory_handle_))
  18     return 0;
  19   renderer_shared_memory_.reset(new base::SharedMemory(
  20       renderer_shared_memory_handle_, true));
  21
  22   return (renderer_shared_memory_->Map(buffer_size))
  23       ? renderer_shared_memory_->memory()
  24       : 0;
  25 }
  26
  27 bool SharedMemorySeqLockReaderBase::FetchFromBuffer(
  28     content::OneWriterSeqLock* seqlock, void* final, void* temp, void* from,
  29     size_t size) {
  30
  31   if (!base::SharedMemory::IsHandleValid(renderer_shared_memory_handle_))
  32     return false;
  33
  34   // Only try to read this many times before failing to avoid waiting here
  35   // very long in case of contention with the writer.
  36   int contention_count = -1;
  37   base::subtle::Atomic32 version;
  38   do {
  39     version = seqlock->ReadBegin();
  40     memcpy(temp, from, size);
  41     ++contention_count;
  42     if (contention_count == kMaximumContentionCount)
  43       break;
  44   } while (seqlock->ReadRetry(version));
  45
  46   if (contention_count >= kMaximumContentionCount) {
  47     // We failed to successfully read, presumably because the hardware
  48     // thread was taking unusually long. Don't copy the data to the output
  49     // buffer, and simply leave what was there before.
  50     return false;
  51   }
  52
  53   // New data was read successfully, copy it into the output buffer.
  54   memcpy(final, temp, size);
  55   return true;
  56 }
  57
  58 }  // namespace internal