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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 | |
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