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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Implementation of an asynchronous lock-free logging system. */
+
+#ifndef mozilla_dom_AsyncLogger_h
+#define mozilla_dom_AsyncLogger_h
+
+#include <atomic>
+#include <thread>
+#include "mozilla/Logging.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/MathAlgorithms.h"
+#include "mozilla/Sprintf.h"
+
+namespace mozilla {
+
+namespace detail {
+
+ // This class implements a lock-free multiple producer single consumer queue of
+ // fixed size log messages, with the following characteristics:
+ // - Unbounded (uses a intrinsic linked list)
+ // - Allocates on Push. Push can be called on any thread.
+ // - Deallocates on Pop. Pop MUST always be called on the same thread for the
+ // life-time of the queue.
+ //
+ // In our scenario, the producer threads are real-time, they can't block. The
+ // consummer thread runs every now and then and empties the queue to a log
+ // file, on disk.
+ //
+ // Having fixed size messages and jemalloc is probably not the fastest, but
+ // allows having a simpler design, we count on the fact that jemalloc will get
+ // the memory from a thread-local source most of the time.
+template<size_t MESSAGE_LENGTH>
+class MPSCQueue
+{
+public:
+ struct Message {
+ Message()
+ {
+ mNext.store(nullptr, std::memory_order_relaxed);
+ }
+ Message(const Message& aMessage) = delete;
+ void operator=(const Message& aMessage) = delete;
+
+ char data[MESSAGE_LENGTH];
+ std::atomic<Message*> mNext;
+ };
+ // Creates a new MPSCQueue. Initially, the queue has a single sentinel node,
+ // pointed to by both mHead and mTail.
+ MPSCQueue()
+ // At construction, the initial message points to nullptr (it has no
+ // successor). It is a sentinel node, that does not contain meaningful
+ // data.
+ : mHead(new Message())
+ , mTail(mHead.load(std::memory_order_relaxed))
+ { }
+
+ ~MPSCQueue()
+ {
+ Message dummy;
+ while (this->Pop(dummy.data)) {}
+ Message* front = mHead.load(std::memory_order_relaxed);
+ delete front;
+ }
+
+ void
+ Push(MPSCQueue<MESSAGE_LENGTH>::Message* aMessage)
+ {
+ // The next two non-commented line are called A and B in this paragraph.
+ // Producer threads i, i-1, etc. are numbered in the order they reached
+ // A in time, thread i being the thread that has reached A first.
+ // Atomically, on line A the new `mHead` is set to be the node that was
+ // just allocated, with strong memory order. From now one, any thread
+ // that reaches A will see that the node just allocated is
+ // effectively the head of the list, and will make itself the new head
+ // of the list.
+ // In a bad case (when thread i executes A and then
+ // is not scheduled for a long time), it is possible that thread i-1 and
+ // subsequent threads create a seemingly disconnected set of nodes, but
+ // they all have the correct value for the next node to set as their
+ // mNext member on their respective stacks (in `prev`), and this is
+ // always correct. When the scheduler resumes, and line B is executed,
+ // the correct linkage is resumed.
+ // Before line B, since mNext for the node the was the last element of
+ // the queue still has an mNext of nullptr, Pop will not see the node
+ // added.
+ // For line A, it's critical to have strong ordering both ways (since
+ // it's going to possibly be read and write repeatidly by multiple
+ // threads)
+ // Line B can have weaker guarantees, it's only going to be written by a
+ // single thread, and we just need to ensure it's read properly by a
+ // single other one.
+ Message* prev = mHead.exchange(aMessage, std::memory_order_acq_rel);
+ prev->mNext.store(aMessage, std::memory_order_release);
+ }
+
+ // Allocates a new node, copy aInput to the new memory location, and pushes
+ // it to the end of the list.
+ void
+ Push(const char aInput[MESSAGE_LENGTH])
+ {
+ // Create a new message, and copy the messages passed on argument to the
+ // new memory location. We are not touching the queue right now. The
+ // successor for this new node is set to be nullptr.
+ Message* msg = new Message();
+ strncpy(msg->data, aInput, MESSAGE_LENGTH);
+
+ Push(msg);
+ }
+
+ // Copy the content of the first message of the queue to aOutput, and
+ // frees the message. Returns true if there was a message, in which case
+ // `aOutput` contains a valid value. If the queue was empty, returns false,
+ // in which case `aOutput` is left untouched.
+ bool
+ Pop(char aOutput[MESSAGE_LENGTH])
+ {
+ // Similarly, in this paragraph, the two following lines are called A
+ // and B, and threads are called thread i, i-1, etc. in order of
+ // execution of line A.
+ // On line A, the first element of the queue is acquired. It is simply a
+ // sentinel node.
+ // On line B, we acquire the node that has the data we want. If B is
+ // null, then only the sentinel node was present in the queue, we can
+ // safely return false.
+ // mTail can be loaded with relaxed ordering, since it's not written nor
+ // read by any other thread (this queue is single consumer).
+ // mNext can be written to by one of the producer, so it's necessary to
+ // ensure those writes are seen, hence the stricter ordering.
+ Message* tail = mTail.load(std::memory_order_relaxed);
+ Message* next = tail->mNext.load(std::memory_order_acquire);
+
+ if (next == nullptr) {
+ return false;
+ }
+
+ strncpy(aOutput, next->data, MESSAGE_LENGTH);
+
+ // Simply shift the queue one node further, so that the sentinel node is
+ // now pointing to the correct most ancient node. It contains stale data,
+ // but this data will never be read again.
+ // It's only necessary to ensure the previous load on this thread is not
+ // reordered past this line, so release ordering is sufficient here.
+ mTail.store(next, std::memory_order_release);
+
+ // This thread is now the only thing that points to `tail`, it can be
+ // safely deleted.
+ delete tail;
+
+ return true;
+ }
+
+private:
+ // An atomic pointer to the most recent message in the queue.
+ std::atomic<Message*> mHead;
+ // An atomic pointer to a sentinel node, that points to the oldest message
+ // in the queue.
+ std::atomic<Message*> mTail;
+
+ MPSCQueue(const MPSCQueue&) = delete;
+ void operator=(const MPSCQueue&) = delete;
+public:
+ // The goal here is to make it easy on the allocator. We pack a pointer in the
+ // message struct, and we still want to do power of two allocations to
+ // minimize allocator slop. The allocation size are going to be constant, so
+ // the allocation is probably going to hit the thread local cache in jemalloc,
+ // making it cheap and, more importantly, lock-free enough.
+ static const size_t MESSAGE_PADDING = sizeof(Message::mNext);
+private:
+ static_assert(IsPowerOfTwo(MESSAGE_LENGTH + MESSAGE_PADDING),
+ "MPSCQueue internal allocations must have a size that is a"
+ "power of two ");
+};
+} // end namespace detail
+
+// This class implements a lock-free asynchronous logger, that outputs to
+// MOZ_LOG.
+// Any thread can use this logger without external synchronization and without
+// being blocked. This log is suitable for use in real-time audio threads.
+// Log formatting is best done externally, this class implements the output
+// mechanism only.
+// This class uses a thread internally, and must be started and stopped
+// manually.
+// If logging is disabled, all the calls are no-op.
+class AsyncLogger
+{
+public:
+ static const uint32_t MAX_MESSAGE_LENGTH = 512 - detail::MPSCQueue<sizeof(void*)>::MESSAGE_PADDING;
+
+ // aLogModuleName is the name of the MOZ_LOG module.
+ explicit AsyncLogger(const char* aLogModuleName)
+ : mThread(nullptr)
+ , mLogModule(aLogModuleName)
+ , mRunning(false)
+ { }
+
+ ~AsyncLogger()
+ {
+ if (Enabled()) {
+ Stop();
+ }
+ }
+
+ void Start()
+ {
+ MOZ_ASSERT(!mRunning, "Double calls to AsyncLogger::Start");
+ if (Enabled()) {
+ mRunning = true;
+ Run();
+ }
+ }
+
+ void Stop()
+ {
+ if (Enabled()) {
+ if (mRunning) {
+ mRunning = false;
+ mThread->join();
+ }
+ } else {
+ MOZ_ASSERT(!mRunning && !mThread);
+ }
+ }
+
+ void Log(const char* format, ...) MOZ_FORMAT_PRINTF(2,3)
+ {
+ if (Enabled()) {
+ auto* msg = new detail::MPSCQueue<MAX_MESSAGE_LENGTH>::Message();
+ va_list args;
+ va_start(args, format);
+ VsprintfLiteral(msg->data, format, args);
+ va_end(args);
+ mMessageQueue.Push(msg);
+ }
+ }
+
+ bool Enabled()
+ {
+ return MOZ_LOG_TEST(mLogModule, mozilla::LogLevel::Verbose);
+ }
+
+private:
+ void Run()
+ {
+ MOZ_ASSERT(Enabled());
+ mThread.reset(new std::thread([this]() {
+ while (mRunning) {
+ char message[MAX_MESSAGE_LENGTH];
+ while (mMessageQueue.Pop(message) && mRunning) {
+ MOZ_LOG(mLogModule, mozilla::LogLevel::Verbose, ("%s", message));
+ }
+ Sleep();
+ }
+ }));
+ }
+
+ void Sleep() { std::this_thread::sleep_for(std::chrono::milliseconds(10)); }
+
+ std::unique_ptr<std::thread> mThread;
+ mozilla::LazyLogModule mLogModule;
+ detail::MPSCQueue<MAX_MESSAGE_LENGTH> mMessageQueue;
+ std::atomic<bool> mRunning;
+};
+
+} // end namespace mozilla
+
+#endif // mozilla_dom_AsyncLogger_h