Add support for Linux eventfd(7) in the UNIX event loop

eventfd(7) uses less resources than a pipe, as it only needs to store a
single 64-bit integer, as opposed to a full buffer.

It was introduced first on Linux version 2.6.22 and glibc 2.7. However,
both the configure-time test and the runtime usage require the use of
EFD_CLOEXEC for thread-safety, so this code will be enabled only for
Linux 2.6.27 and up as well as glibc 2.9 and up.

Change-Id: Ic7e10b28d7b1d4ca24be614ed84055c4429a68e4
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>

config.tests/unix/eventfd/eventfd.pro

@@ -0,0 +1,3 @@
+SOURCES = main.cpp
+CONFIG -= qt dylib
+mac:CONFIG -= app_bundle

config.tests/unix/eventfd/main.cpp

@@ -0,0 +1,51 @@
+/****************************************************************************
+**
+** Copyright (C) 2012 Intel Corporation
+** Contact: http://www.qt-project.org/legal
+**
+** This file is part of the config.tests 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 Digia.  For licensing terms and
+** conditions see http://qt.digia.com/licensing.  For further information
+** use the contact form at http://qt.digia.com/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 2.1 as published by the Free Software
+** Foundation and appearing in the file LICENSE.LGPL included in the
+** packaging of this file.  Please review the following information to
+** ensure the GNU Lesser General Public License version 2.1 requirements
+** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
+**
+** In addition, as a special exception, Digia gives you certain additional
+** rights.  These rights are described in the Digia Qt LGPL Exception
+** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3.0 as published by the Free Software
+** Foundation and appearing in the file LICENSE.GPL included in the
+** packaging of this file.  Please review the following information to
+** ensure the GNU General Public License version 3.0 requirements will be
+** met: http://www.gnu.org/copyleft/gpl.html.
+**
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#include <sys/eventfd.h>
+
+int main()
+{
+    eventfd_t value;
+    int fd = eventfd(0, EFD_CLOEXEC);
+    eventfd_read(fd, &value);
+    eventfd_write(fd, value);
+    return 0;
+}

configure

@@ -842,6 +842,7 @@ CFG_GETADDRINFO=auto
 CFG_IPV6IFNAME=auto
 CFG_GETIFADDRS=auto
 CFG_INOTIFY=auto
+CFG_EVENTFD=auto
 CFG_RPATH=yes
 CFG_FRAMEWORK=auto
 CFG_MAC_HARFBUZZ=no
@@ -5099,6 +5100,23 @@ if [ "$CFG_INOTIFY" != "no" ]; then
     fi
 fi
 
+# find if the platform provides eventfd
+if [ "$CFG_EVENTFD" != "no" ]; then
+    if compileTest unix/eventfd "eventfd"; then
+        CFG_EVENTFD=yes
+    else
+        if [ "$CFG_EVENTFD" = "yes" ] && [ "$CFG_CONFIGURE_EXIT_ON_ERROR" = "yes" ]; then
+            echo "eventfd support cannot be enabled due to functionality tests!"
+            echo " Turn on verbose messaging (-v) to $0 to see the final report."
+            echo " If you believe this message is in error you may use the continue"
+            echo " switch (-continue) to $0 to continue."
+            exit 101
+        else
+            CFG_EVENTFD=no
+        fi
+    fi
+fi
+
 # find if the platform provides if_nametoindex (ipv6 interface name support)
 if [ "$CFG_IPV6IFNAME" != "no" ]; then
     if compileTest unix/ipv6ifname "IPv6 interface name"; then
@@ -5386,6 +5404,9 @@ fi
 if [ "$CFG_INOTIFY" = "yes" ]; then
     QT_CONFIG="$QT_CONFIG inotify"
 fi
+if [ "$CFG_EVENTFD" = "yes" ]; then
+    QT_CONFIG="$QT_CONFIG eventfd"
+fi
 if [ "$CFG_LIBJPEG" = "no" ]; then
     CFG_JPEG="no"
 elif [ "$CFG_LIBJPEG" = "system" ]; then
@@ -5862,6 +5883,7 @@ QMakeVar set sql-plugins "$SQL_PLUGINS"
 [ "$CFG_IPV6IFNAME" = "no" ] && QCONFIG_FLAGS="$QCONFIG_FLAGS QT_NO_IPV6IFNAME"
 [ "$CFG_GETIFADDRS" = "no" ] && QCONFIG_FLAGS="$QCONFIG_FLAGS QT_NO_GETIFADDRS"
 [ "$CFG_INOTIFY" = "no" ]    && QCONFIG_FLAGS="$QCONFIG_FLAGS QT_NO_INOTIFY"
+[ "$CFG_EVENTFD" = "no" ]    && QCONFIG_FLAGS="$QCONFIG_FLAGS QT_NO_EVENTFD"
 [ "$CFG_NIS" = "no" ]        && QCONFIG_FLAGS="$QCONFIG_FLAGS QT_NO_NIS"
 [ "$CFG_OPENSSL" = "no" ]    && QCONFIG_FLAGS="$QCONFIG_FLAGS QT_NO_OPENSSL QT_NO_SSL"
 [ "$CFG_OPENSSL" = "linked" ]&& QCONFIG_FLAGS="$QCONFIG_FLAGS QT_LINKED_OPENSSL"

src/corelib/kernel/qeventdispatcher_unix.cpp

@@ -56,6 +56,10 @@
 #include <stdio.h>
 #include <stdlib.h>
 
+#ifndef QT_NO_EVENTFD
+#  include <sys/eventfd.h>
+#endif
+
 // VxWorks doesn't correctly set the _POSIX_... options
 #if defined(Q_OS_VXWORKS)
 #  if defined(_POSIX_MONOTONIC_CLOCK) && (_POSIX_MONOTONIC_CLOCK <= 0)
@@ -127,6 +131,12 @@ QEventDispatcherUNIXPrivate::QEventDispatcherUNIXPrivate()
         }
     }
 #else
+#  ifndef QT_NO_EVENTFD
+    thread_pipe[0] = eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC);
+    if (thread_pipe[0] != -1)
+        thread_pipe[1] = -1;
+    else // fall through the next "if"
+#  endif
     if (qt_safe_pipe(thread_pipe, O_NONBLOCK) == -1) {
         perror("QEventDispatcherUNIXPrivate(): Unable to create thread pipe");
         pipefail = true;
@@ -155,7 +165,8 @@ QEventDispatcherUNIXPrivate::~QEventDispatcherUNIXPrivate()
 #else
     // cleanup the common parts of the event loop
     close(thread_pipe[0]);
-    close(thread_pipe[1]);
+    if (thread_pipe[1] != -1)
+        close(thread_pipe[1]);
 #endif
 
     // cleanup timers
@@ -279,9 +290,18 @@ int QEventDispatcherUNIXPrivate::processThreadWakeUp(int nsel)
         ::read(thread_pipe[0], c, sizeof(c));
         ::ioctl(thread_pipe[0], FIOFLUSH, 0);
 #else
-        char c[16];
-        while (::read(thread_pipe[0], c, sizeof(c)) > 0)
-            ;
+#  ifndef QT_NO_EVENTFD
+        if (thread_pipe[1] == -1) {
+            // eventfd
+            eventfd_t value;
+            eventfd_read(thread_pipe[0], &value);
+        } else
+#  endif
+        {
+            char c[16];
+            while (::read(thread_pipe[0], c, sizeof(c)) > 0) {
+            }
+        }
 #endif
         if (!wakeUps.testAndSetRelease(1, 0)) {
             // hopefully, this is dead code
@@ -630,6 +650,15 @@ void QEventDispatcherUNIX::wakeUp()
 {
     Q_D(QEventDispatcherUNIX);
     if (d->wakeUps.testAndSetAcquire(0, 1)) {
+#ifndef QT_NO_EVENTFD
+        if (d->thread_pipe[1] == -1) {
+            // eventfd
+            eventfd_t value = 1;
+            int ret;
+            EINTR_LOOP(ret, eventfd_write(d->thread_pipe[0], value));
+            return;
+        }
+#endif
         char c = 0;
         qt_safe_write( d->thread_pipe[1], &c, 1 );
     }

src/corelib/kernel/qeventdispatcher_unix_p.h

@@ -148,6 +148,9 @@ public:
     virtual int processThreadWakeUp(int nsel);
 
     bool mainThread;
+
+    // note for eventfd(7) support:
+    // if thread_pipe[1] is -1, then eventfd(7) is in use and is stored in thread_pipe[0]
     int thread_pipe[2];
 
     // highest fd for all socket notifiers