add futex semaphore support derived from hutchinson port

Author: frankeh

configure.ac

@@ -276,6 +276,8 @@ AC_CHECK_FUNC([sem_init],
   [have_sem_init=true], [have_sem_init=false]
 )
 
+AC_CHECK_HEADER(linux/futex.h, [have_futex=true], [have_futex=false])
+
 #
 # We support both Mach semaphores and POSIX semaphores; if the former are
 # available, prefer them.
@@ -284,6 +286,9 @@ AC_MSG_CHECKING([what semaphore type to use]);
 AS_IF([test "x$have_mach" = "xtrue"],
   [AC_DEFINE(USE_MACH_SEM, 1, [Define to use Mach semaphores])
     AC_MSG_RESULT([Mach semaphores])],
+  [test "x$have_futex" = "xtrue"],
+  [AC_DEFINE(USE_FUTEX_SEM, 1, [Define to use Futex semaphores])
+    AC_MSG_RESULT([Futex semaphores])],
   [test "x$have_sem_init" = "xtrue"],
   [AC_DEFINE(USE_POSIX_SEM, 1, [Define to use POSIX semaphores])
     AC_MSG_RESULT([POSIX semaphores])],

src/internal.h

@@ -230,7 +230,11 @@ DISPATCH_EXPORT DISPATCH_NOTHROW void dispatch_atfork_child(void);
 #include <search.h>
 #if USE_POSIX_SEM
 #include <semaphore.h>
+#elif USE_FUTEX_SEM
+#include <sys/syscall.h>
+#include <linux/futex.h>
 #endif
+
 #include <signal.h>
 #include <stdarg.h>
 #include <stdbool.h>
@@ -242,6 +246,34 @@ DISPATCH_EXPORT DISPATCH_NOTHROW void dispatch_atfork_child(void);
 #include <unistd.h>
 #endif
 
+#if USE_POSIX_SEM
+#include <semaphore.h>
+#elif USE_FUTEX_SEM
+#include <sys/syscall.h>
+#include <linux/futex.h>
+#endif
+
+#if defined(__APPLE__)
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+#define DISPATCH_LITTLE_ENDIAN
+#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
+#define DISPATCH_BIG_ENDIAN
+#endif
+
+#elif defined(__linux__)
+#include <endian.h>
+#if __BYTE_ORDER == __LITTLE_ENDIAN
+#define DISPATCH_LITTLE_ENDIAN
+#elif __BYTE_ORDER == __BIG_ENDIAN
+#define DISPATCH_BIG_ENDIAN
+#endif
+
+#endif  /* defined(__APPLE__) */
+
+#if !(defined(DISPATCH_BIG_ENDIAN) || defined(DISPATCH_LITTLE_ENDIAN))
+#error "Unable to determine platform endianness."
+#endif
+
 #ifndef __has_builtin
 #define __has_builtin(x) 0
 #endif

src/queue.c

@@ -879,6 +879,9 @@ _dispatch_root_queue_init_pthread_pool(dispatch_root_queue_context_t qc,
 	/* XXXRW: POSIX semaphores don't support LIFO? */
 	int ret = sem_init(&(pqc->dpq_thread_mediator.dsema_sem), 0, 0);
 	(void)dispatch_assume_zero(ret);
+#elif USE_FUTEX_SEM
+	int ret = _dispatch_futex_init(&(pqc->dpq_thread_mediator.dsema_futex));
+	(void)dispatch_assume_zero(ret);
 #endif
 }
 #endif // DISPATCH_USE_PTHREAD_POOL