Use direct futexes for MOD target write lock

Signed-off-by: falkTX <falktx@falktx.com>

doc/MODDEVICES.md

@@ -41,4 +41,5 @@ There are a few critical things to take into consideration for this release:
 you can and should use multiple instances when there is no desktop connection though
 - The MOD unit must be connected over USB, so that the 192.168.51.1 IP is reachable
 - The Audio File, Carla and Ildaeil modules are not available in MOD builds
+- Lights and meters from the Cardinal MOD side are not transmitted back to the desktop side
 - Compared to desktop, MOD builds are not as fast, so do not expect to load big patches

include/mutex.hpp

@@ -19,13 +19,26 @@
 
 #include <pthread.h>
 
+#ifdef __MOD_DEVICES__
+#include <linux/futex.h>
+#include <sys/time.h>
+#include <errno.h>
+#include <syscall.h>
+#include <unistd.h>
+#endif
+
 /* replace Rack's mutex with our own custom one, which can do priority inversion. */
 
 namespace rack {
 
 
 struct SharedMutex {
-    pthread_mutex_t readLock, writeLock;
+    pthread_mutex_t readLock;
+#ifdef __MOD_DEVICES__
+    int writeLock;
+#else
+    pthread_mutex_t writeLock;
+#endif
 
     SharedMutex() noexcept {
         pthread_mutexattr_t attr;
@@ -35,27 +48,52 @@ struct SharedMutex {
         pthread_mutex_init(&readLock, &attr);
         pthread_mutexattr_destroy(&attr);
 
+#ifdef __MOD_DEVICES__
+        writeLock = 1;
+#else
         pthread_mutexattr_t attr2;
         pthread_mutexattr_init(&attr2);
         pthread_mutexattr_setprotocol(&attr2, PTHREAD_PRIO_NONE);
         pthread_mutexattr_settype(&attr2, PTHREAD_MUTEX_NORMAL);
         pthread_mutex_init(&writeLock, &attr2);
         pthread_mutexattr_destroy(&attr2);
+#endif
     }
 
     ~SharedMutex() noexcept {
         pthread_mutex_destroy(&readLock);
+#ifndef __MOD_DEVICES__
         pthread_mutex_destroy(&writeLock);
+#endif
     }
 
     // for std::lock_guard usage, writers lock
     void lock() noexcept {
+#ifdef __MOD_DEVICES__
+        for (;;)
+        {
+            if (__sync_bool_compare_and_swap(&writeLock, 1, 0))
+                return;
+
+            if (syscall(__NR_futex, &writeLock, FUTEX_WAIT_PRIVATE, 0, nullptr, nullptr, 0) != 0)
+            {
+                if (errno != EAGAIN && errno != EINTR)
+                    return;
+            }
+        }
+#else
         pthread_mutex_lock(&writeLock);
+#endif
         pthread_mutex_lock(&readLock);
     }
     void unlock() noexcept {
         pthread_mutex_unlock(&readLock);
+#ifdef __MOD_DEVICES__
+        if (__sync_bool_compare_and_swap(&writeLock, 0, 1))
+            syscall(__NR_futex, &writeLock, FUTEX_WAKE_PRIVATE, 1, nullptr, nullptr, 0);
+#else
         pthread_mutex_unlock(&writeLock);
+#endif
     }
 
     // for SharedLock usage, readers lock