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 59
