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// Copyright 2013 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.
#include "content/renderer/shared_memory_seqlock_reader.h"
namespace content {
namespace internal {
SharedMemorySeqLockReaderBase::SharedMemorySeqLockReaderBase() { }
SharedMemorySeqLockReaderBase::~SharedMemorySeqLockReaderBase() { }
void*
SharedMemorySeqLockReaderBase::InitializeSharedMemory(
base::SharedMemoryHandle shared_memory_handle, size_t buffer_size) {
renderer_shared_memory_handle_ = shared_memory_handle;
if (!base::SharedMemory::IsHandleValid(renderer_shared_memory_handle_))
return 0;
renderer_shared_memory_.reset(new base::SharedMemory(
renderer_shared_memory_handle_, true));
return (renderer_shared_memory_->Map(buffer_size))
? renderer_shared_memory_->memory()
: 0;
}
bool SharedMemorySeqLockReaderBase::FetchFromBuffer(
content::OneWriterSeqLock* seqlock, void* final, void* temp, void* from,
size_t size) {
if (!base::SharedMemory::IsHandleValid(renderer_shared_memory_handle_))
return false;
// Only try to read this many times before failing to avoid waiting here
// very long in case of contention with the writer.
int contention_count = -1;
base::subtle::Atomic32 version;
do {
version = seqlock->ReadBegin();
memcpy(temp, from, size);
++contention_count;
if (contention_count == kMaximumContentionCount)
break;
} while (seqlock->ReadRetry(version));
if (contention_count >= kMaximumContentionCount) {
// We failed to successfully read, presumably because the hardware
// thread was taking unusually long. Don't copy the data to the output
// buffer, and simply leave what was there before.
return false;
}
// New data was read successfully, copy it into the output buffer.
memcpy(final, temp, size);
return true;
}
} // namespace internal
} // namespace content
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