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
SharedMemorySeqLockReaderBase()9 SharedMemorySeqLockReaderBase::SharedMemorySeqLockReaderBase() { }
10
~SharedMemorySeqLockReaderBase()11 SharedMemorySeqLockReaderBase::~SharedMemorySeqLockReaderBase() { }
12
13 void*
InitializeSharedMemory(base::SharedMemoryHandle shared_memory_handle,size_t buffer_size)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
FetchFromBuffer(content::OneWriterSeqLock * seqlock,void * final,void * temp,void * from,size_t size)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
59