Implement Futex semaphore

libs/pbd/pbd/semutils.h

@@ -21,15 +21,19 @@
 #ifndef __pbd_semutils_h__
 #define __pbd_semutils_h__
 
+#define USE_FUTEX_SEMAPHORE
+
 #if (defined PLATFORM_WINDOWS && !defined USE_PTW32_SEMAPHORE)
 #define WINDOWS_SEMAPHORE 1
 #endif
 
 #ifdef WINDOWS_SEMAPHORE
-#include <windows.h>
+# include <windows.h>
+#elif defined USE_FUTEX_SEMAPHORE
+# include <atomic>
 #else
-#include <pthread.h>
-#include <semaphore.h>
+# include <pthread.h>
+# include <semaphore.h>
 #endif
 
 #include "pbd/libpbd_visibility.h"
@@ -44,6 +48,8 @@ class LIBPBD_API Semaphore {
 #elif __APPLE__
 	sem_t* _sem;
 	sem_t* ptr_to_sem() const { return _sem; }
+#elif defined USE_FUTEX_SEMAPHORE
+	std::atomic<unsigned int> _value;
 #else
 	mutable sem_t _sem;
 	sem_t* ptr_to_sem() const { return &_sem; }
@@ -53,7 +59,7 @@ class LIBPBD_API Semaphore {
 	Semaphore (const char* name, int val);
 	~Semaphore ();
 
-#ifdef WINDOWS_SEMAPHORE
+#if defined WINDOWS_SEMAPHORE || defined USE_FUTEX_SEMAPHORE
 
 	int signal ();
 	int wait ();

libs/pbd/semutils.cc

@@ -20,6 +20,13 @@
 #include "pbd/semutils.h"
 #include "pbd/failed_constructor.h"
 
+#ifdef USE_FUTEX_SEMAPHORE
+#include <errno.h>
+#include <linux/futex.h>
+#include <syscall.h>
+#include <unistd.h>
+#endif
+
 using namespace PBD;
 
 Semaphore::Semaphore (const char* name, int val)
@@ -41,6 +48,9 @@ Semaphore::Semaphore (const char* name, int val)
 		throw failed_constructor ();
 	}
 
+#elif defined USE_FUTEX_SEMAPHORE
+	(void)name; /* stop warning */
+	_value = val;
 #else
 	(void) name; /* stop gcc warning on !Apple systems */
 
@@ -88,4 +98,45 @@ Semaphore::reset ()
 	return rv;
 }
 
+#elif defined USE_FUTEX_SEMAPHORE
+
+int
+Semaphore::signal ()
+{
+	std::atomic_fetch_add_explicit (&_value, 1, std::memory_order_release);
+	return syscall (__NR_futex, &_value, FUTEX_WAKE_PRIVATE, 1, NULL, NULL, 0);
+}
+
+int
+Semaphore::wait ()
+{
+	unsigned int value = 1;
+	while (!std::atomic_compare_exchange_weak_explicit (&_value, &value, value - 1, std::memory_order_acquire, std::memory_order_relaxed)) {
+		if (value == 0) {
+			if (syscall (__NR_futex, &_value, FUTEX_WAIT_PRIVATE, 0, NULL, NULL, 0)) {
+				if (errno != EAGAIN && errno != EINTR) {
+					return 1;
+				}
+			}
+			value = 1;
+		}
+	}
+	return 0;
+}
+
+int
+Semaphore::reset ()
+{
+	int rv = 0;
+	unsigned int value = 1;
+
+	while (!std::atomic_compare_exchange_weak_explicit (&_value, &value, value - 1, std::memory_order_acquire, std::memory_order_relaxed)) {
+		if (value == 0) {
+			break;
+		}
+		++rv;
+	}
+	return rv;
+}
+
 #endif