xcb: lock-free event processing

For details how this works refer to the documentation in the patch.

The follow-up patches will switch to calling processXcbEvents() on every
event loop iteration. With the existing code that would mean frequent
locking of shared data (event queue). Acquiring a lock is fast, but
lock contention isn't. To avoid potential problems, reimplement xcb event
processing to be lock-free. Besides theoretical performance benefits,
this definitally improves code readability in qxcbconnection.cpp. Thanks
to Mikhail Svetkin for questioning the design of the existing code.

Done-with: Mikhail Svetkin <mikhail.svetkin@qt.io>
Change-Id: I935f2b6ca802580f5c80205aef7b2f9afc172d26
Reviewed-by: Mikhail Svetkin <mikhail.svetkin@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 160 insertions, 0 deletions

diff --git a/src/plugins/platforms/xcb/qxcbeventqueue.h b/src/plugins/platforms/xcb/qxcbeventqueue.h
new file mode 100644
index 0000000000..5b0d8bf3d6
--- /dev/null
+++ b/src/plugins/platforms/xcb/qxcbeventqueue.h
@@ -0,0 +1,160 @@
+/****************************************************************************
+**
+** Copyright (C) 2018 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the QtCore module of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU Lesser General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU Lesser
+** General Public License version 3 as published by the Free Software
+** Foundation and appearing in the file LICENSE.LGPL3 included in the
+** packaging of this file. Please review the following information to
+** ensure the GNU Lesser General Public License version 3 requirements
+** will be met: https://www.gnu.org/licenses/lgpl-3.0.html.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 2.0 or (at your option) the GNU General
+** Public license version 3 or any later version approved by the KDE Free
+** Qt Foundation. The licenses are as published by the Free Software
+** Foundation and appearing in the file LICENSE.GPL2 and LICENSE.GPL3
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-2.0.html and
+** https://www.gnu.org/licenses/gpl-3.0.html.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+#ifndef QXCBEVENTQUEUE_H
+#define QXCBEVENTQUEUE_H
+
+#include <QtCore/QThread>
+#include <QtCore/QHash>
+
+#include <xcb/xcb.h>
+
+#include <atomic>
+
+QT_BEGIN_NAMESPACE
+
+struct QXcbEventNode {
+    QXcbEventNode(xcb_generic_event_t *e = nullptr)
+        : event(e) { }
+
+    xcb_generic_event_t *event;
+    QXcbEventNode *next = nullptr;
+    bool fromHeap = false;
+};
+
+class QXcbConnection;
+class QAbstractEventDispatcher;
+
+class QXcbEventQueue : public QThread
+{
+    Q_OBJECT
+public:
+    QXcbEventQueue(QXcbConnection *connection);
+    ~QXcbEventQueue();
+
+    enum { PoolSize = 100 }; // 2.4 kB with 100 nodes
+
+    enum PeekOption {
+        PeekDefault = 0, // see qx11info_x11.h for docs
+        PeekFromCachedIndex = 1,
+        PeekRetainMatch = 2,
+        PeekRemoveMatch = 3,
+        PeekRemoveMatchContinue = 4
+    };
+    Q_DECLARE_FLAGS(PeekOptions, PeekOption)
+
+    void run() override;
+
+    void registerEventDispatcher(QAbstractEventDispatcher *dispatcher);
+
+    bool isEmpty() const { return m_head == m_flushedTail && !m_head->event; }
+    xcb_generic_event_t *takeFirst();
+    void flushBufferedEvents();
+
+    // ### peek() and peekEventQueue() could be unified. Note that peekEventQueue()
+    // is public API exposed via QX11Extras/QX11Info.
+    template<typename Peeker>
+    xcb_generic_event_t *peek(Peeker &&peeker) {
+        return peek(PeekRemoveMatch, std::forward<Peeker>(peeker));
+    }
+    template<typename Peeker>
+    inline xcb_generic_event_t *peek(PeekOption config, Peeker &&peeker);
+
+    qint32 generatePeekerId();
+    bool removePeekerId(qint32 peekerId);
+
+    using PeekerCallback = bool (*)(xcb_generic_event_t *event, void *peekerData);
+    bool peekEventQueue(PeekerCallback peeker, void *peekerData = nullptr,
+                        PeekOptions option = PeekDefault, qint32 peekerId = -1);
+signals:
+    void eventsPending();
+
+private:
+    QXcbEventNode *qXcbEventNodeFactory(xcb_generic_event_t *event);
+    void dequeueNode();
+
+    void sendCloseConnectionEvent() const;
+    bool isCloseConnectionEvent(const xcb_generic_event_t *event);
+
+    QXcbEventNode *m_head = nullptr;
+    QXcbEventNode *m_flushedTail = nullptr;
+    std::atomic<QXcbEventNode *> m_tail { nullptr };
+    std::atomic_uint m_nodesRestored { 0 };
+
+    QXcbConnection *m_connection = nullptr;
+    bool m_closeConnectionDetected = false;
+
+    uint m_freeNodes = PoolSize;
+    uint m_poolIndex = 0;
+
+    qint32 m_peekerIdSource = 0;
+    bool m_queueModified = false;
+    bool m_peekerIndexCacheDirty = false;
+    QHash<qint32, QXcbEventNode *> m_peekerToNode;
+
+    // debug stats
+    quint64 m_nodesOnHeap = 0;
+};
+
+template<typename Peeker>
+xcb_generic_event_t *QXcbEventQueue::peek(PeekOption option, Peeker &&peeker)
+{
+    flushBufferedEvents();
+    if (isEmpty())
+        return nullptr;
+
+    QXcbEventNode *node = m_head;
+    do {
+        xcb_generic_event_t *event = node->event;
+        if (event && peeker(event, event->response_type & ~0x80)) {
+            if (option == PeekRemoveMatch || option == PeekRemoveMatchContinue)
+                node->event = nullptr;
+            if (option != PeekRemoveMatchContinue)
+                return event;
+        }
+        if (node == m_flushedTail)
+            break;
+        node = node->next;
+    } while (true);
+
+    return nullptr;
+}
+
+QT_END_NAMESPACE
+
+#endif