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