--- /dev/null
+From 49b59e5ac4428a6a99a85d699c3662f96efc4e9d Mon Sep 17 00:00:00 2001
+From: Ryan Lortie <desrt@desrt.ca>
+Date: Tue, 10 Jun 2014 08:28:32 -0400
+Subject: [PATCH] GLib: implement GMutex natively on Linux
+
+If we have futex(2) then we can implement GMutex natively and gain a
+substantial performance increase (vs. using pthreads).
+
+This also avoids the need to allocate an extra structure in memory when
+using GMutex or GCond: we can use the structure directly.
+
+The main reason for the increase in performance is that our
+implementation can be made more simple: we don't need to support the
+array of options on pthread_mutex_t (which includes the possibility, for
+example, of being recursive).
+
+The result is a ~30% improvement in uncontended cases and a much larger
+increase (3 to 4 times) in contended cases for a simple testcase.
+
+https://bugzilla.gnome.org/show_bug.cgi?id=731986
+---
+ glib/gthread-posix.c | 208 ++++++++++++++++++++++++++++++++++++++++++++++++++-
+ 1 file changed, 207 insertions(+), 1 deletion(-)
+
+diff --git a/glib/gthread-posix.c b/glib/gthread-posix.c
+index 6f5a606..f7d5d8a 100644
+--- a/glib/gthread-posix.c
++++ b/glib/gthread-posix.c
+@@ -66,6 +66,11 @@
+ #include <windows.h>
+ #endif
+
++/* clang defines __ATOMIC_SEQ_CST but doesn't support the GCC extension */
++#if defined(HAVE_FUTEX) && defined(__ATOMIC_SEQ_CST) && !defined(__clang__)
++#define USE_NATIVE_MUTEX
++#endif
++
+ static void
+ g_thread_abort (gint status,
+ const gchar *function)
+@@ -77,6 +82,8 @@ g_thread_abort (gint status,
+
+ /* {{{1 GMutex */
+
++#if !defined(USE_NATIVE_MUTEX)
++
+ static pthread_mutex_t *
+ g_mutex_impl_new (void)
+ {
+@@ -258,6 +265,8 @@ g_mutex_trylock (GMutex *mutex)
+ return FALSE;
+ }
+
++#endif /* !defined(USE_NATIVE_MUTEX) */
++
+ /* {{{1 GRecMutex */
+
+ static pthread_mutex_t *
+@@ -631,6 +640,8 @@ g_rw_lock_reader_unlock (GRWLock *rw_lock)
+
+ /* {{{1 GCond */
+
++#if !defined(USE_NATIVE_MUTEX)
++
+ static pthread_cond_t *
+ g_cond_impl_new (void)
+ {
+@@ -902,6 +913,8 @@ g_cond_wait_until (GCond *cond,
+ return FALSE;
+ }
+
++#endif /* defined(USE_NATIVE_MUTEX) */
++
+ /* {{{1 GPrivate */
+
+ /**
+@@ -1219,5 +1232,198 @@ g_system_thread_set_name (const gchar *name)
+ #endif
+ }
+
+-/* {{{1 Epilogue */
++/* {{{1 GMutex and GCond futex implementation */
++
++#if defined(USE_NATIVE_MUTEX)
++
++#include <linux/futex.h>
++#include <sys/syscall.h>
++
++/* We should expand the set of operations available in gatomic once we
++ * have better C11 support in GCC in common distributions (ie: 4.9).
++ *
++ * Before then, let's define a couple of useful things for our own
++ * purposes...
++ */
++
++#define exchange_acquire(ptr, new) \
++ __atomic_exchange_4((ptr), (new), __ATOMIC_ACQUIRE)
++#define compare_exchange_acquire(ptr, old, new) \
++ __atomic_compare_exchange_4((ptr), (old), (new), 0, __ATOMIC_ACQUIRE, __ATOMIC_RELAXED)
++
++#define exchange_release(ptr, new) \
++ __atomic_exchange_4((ptr), (new), __ATOMIC_RELEASE)
++#define store_release(ptr, new) \
++ __atomic_store_4((ptr), (new), __ATOMIC_RELEASE)
++
++/* Our strategy for the mutex is pretty simple:
++ *
++ * 0: not in use
++ *
++ * 1: acquired by one thread only, no contention
++ *
++ * > 1: contended
++ *
++ *
++ * As such, attempting to acquire the lock should involve an increment.
++ * If we find that the previous value was 0 then we can return
++ * immediately.
++ *
++ * On unlock, we always store 0 to indicate that the lock is available.
++ * If the value there was 1 before then we didn't have contention and
++ * can return immediately. If the value was something other than 1 then
++ * we have the contended case and need to wake a waiter.
++ *
++ * If it was not 0 then there is another thread holding it and we must
++ * wait. We must always ensure that we mark a value >1 while we are
++ * waiting in order to instruct the holder to do a wake operation on
++ * unlock.
++ */
++
++void
++g_mutex_init (GMutex *mutex)
++{
++ mutex->i[0] = 0;
++}
++
++void
++g_mutex_clear (GMutex *mutex)
++{
++}
++
++static void __attribute__((noinline))
++g_mutex_lock_slowpath (GMutex *mutex)
++{
++ /* Set to 2 to indicate contention. If it was zero before then we
++ * just acquired the lock.
++ *
++ * Otherwise, sleep for as long as the 2 remains...
++ */
++ while (exchange_acquire (&mutex->i[0], 2) != 0)
++ syscall (__NR_futex, &mutex->i[0], (gsize) FUTEX_WAIT, (gsize) 2, NULL);
++}
++
++static void __attribute__((noinline))
++g_mutex_unlock_slowpath (GMutex *mutex)
++{
++ /* We seem to get better code for the uncontended case by splitting
++ * out this call...
++ */
++ syscall (__NR_futex, &mutex->i[0], (gsize) FUTEX_WAKE, (gsize) 1, NULL);
++}
++
++void
++g_mutex_lock (GMutex *mutex)
++{
++ /* 0 -> 1 and we're done. Anything else, and we need to wait... */
++ if G_UNLIKELY (g_atomic_int_add (&mutex->i[0], 1) != 0)
++ g_mutex_lock_slowpath (mutex);
++}
++
++void
++g_mutex_unlock (GMutex *mutex)
++{
++ /* 1-> 0 and we're done. Anything else and we need to signal... */
++ if G_UNLIKELY (exchange_release (&mutex->i[0], 0) != 1)
++ g_mutex_unlock_slowpath (mutex);
++}
++
++gboolean
++g_mutex_trylock (GMutex *mutex)
++{
++ guint zero = 0;
++
++ /* We don't want to touch the value at all unless we can move it from
++ * exactly 0 to 1.
++ */
++ return compare_exchange_acquire (&mutex->i[0], &zero, 1);
++}
++
++/* Condition variables are implemented in a rather simple way as well.
++ * In many ways, futex() as an abstraction is even more ideally suited
++ * to condition variables than it is to mutexes.
++ *
++ * We store a generation counter. We sample it with the lock held and
++ * unlock before sleeping on the futex.
++ *
++ * Signalling simply involves increasing the counter and making the
++ * appropriate futex call.
++ *
++ * The only thing that is the slightest bit complicated is timed waits
++ * because we must convert our absolute time to relative.
++ */
++
++void
++g_cond_init (GCond *cond)
++{
++ cond->i[0] = 0;
++}
++
++void
++g_cond_clear (GCond *cond)
++{
++}
++
++void
++g_cond_wait (GCond *cond,
++ GMutex *mutex)
++{
++ guint sampled = g_atomic_int_get (&cond->i[0]);
++
++ g_mutex_unlock (mutex);
++ syscall (__NR_futex, &cond->i[0], (gsize) FUTEX_WAIT, (gsize) sampled, NULL);
++ g_mutex_lock (mutex);
++}
++
++void
++g_cond_signal (GCond *cond)
++{
++ g_atomic_int_inc (&cond->i[0]);
++
++ syscall (__NR_futex, &cond->i[0], (gsize) FUTEX_WAKE, (gsize) 1, NULL);
++}
++
++void
++g_cond_broadcast (GCond *cond)
++{
++ g_atomic_int_inc (&cond->i[0]);
++
++ syscall (__NR_futex, &cond->i[0], (gsize) FUTEX_WAKE, (gsize) INT_MAX, NULL);
++}
++
++gboolean
++g_cond_wait_until (GCond *cond,
++ GMutex *mutex,
++ gint64 end_time)
++{
++ struct timespec now;
++ struct timespec span;
++ guint sampled;
++
++ if (end_time < 0)
++ return FALSE;
++
++ clock_gettime (CLOCK_MONOTONIC, &now);
++ span.tv_sec = (end_time / 1000000) - now.tv_sec;
++ span.tv_nsec = ((end_time % 1000000) * 1000) - now.tv_nsec;
++ if (span.tv_nsec < 0)
++ {
++ span.tv_nsec += 1000000000;
++ span.tv_sec--;
++ }
++
++ if (span.tv_sec < 0)
++ return FALSE;
++
++ sampled = cond->i[0];
++ g_mutex_unlock (mutex);
++ syscall (__NR_futex, &cond->i[0], (gsize) FUTEX_WAIT, (gsize) sampled, &span);
++ g_mutex_lock (mutex);
++
++ return TRUE;
++}
++
++#endif
++
++ /* {{{1 Epilogue */
+ /* vim:set foldmethod=marker: */
+--
+1.8.1.4
+
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