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