[kernel] Add (wip) futex syscalls

Add the syscalls j6_futex_wait and j6_futex_wake. Currently marking this
as WIP as they need more testing.

Added to support futexes:
- vm_area and vm_space support for looking up physical address for a
  virtual address
- libj6 mutex implementation using futex system calls

src/libraries/j6/j6.module

@@ -6,6 +6,7 @@ j6 = module("j6",
     sources = [
         "init.cpp",
         "init.s",
+        "mutex.cpp",
         "protocol_ids.cpp",
         "syscalls.s.cog",
         "sysconf.cpp.cog",
@@ -17,6 +18,7 @@ j6 = module("j6",
         "j6/errors.h",
         "j6/flags.h",
         "j6/init.h",
+        "j6/mutex.hh",
         "j6/protocols.h",
         "j6/protocols/service_locator.h",
         "j6/syscalls.h.cog",

src/libraries/j6/j6/mutex.hh

@@ -0,0 +1,42 @@
+#pragma once
+/// \file mutex.hh
+/// High level mutex interface
+
+// The kernel depends on libj6 for some shared code,
+// but should not include the user-specific code.
+#ifndef __j6kernel
+
+#include <stdint.h>
+
+namespace j6 {
+
+class mutex
+{
+public:
+    mutex() : m_state(0) {}
+
+    void lock();
+    void unlock();
+
+private:
+    uint32_t m_state;
+};
+
+class scoped_lock
+{
+public:
+    inline scoped_lock(mutex &m) : m_mutex {m} {
+        m_mutex.lock();
+    }
+
+    inline ~scoped_lock() {
+        m_mutex.unlock();
+    }
+
+private:
+    mutex &m_mutex;
+};
+
+} // namespace j6
+
+#endif // __j6kernel

src/libraries/j6/mutex.cpp

@@ -0,0 +1,36 @@
+// The kernel depends on libj6 for some shared code,
+// but should not include the user-specific code.
+#ifndef __j6kernel
+
+#include <j6/mutex.hh>
+#include <j6/syscalls.h>
+
+namespace j6 {
+
+void
+mutex::lock()
+{
+    uint32_t lock = 0;
+    if ((lock = __sync_val_compare_and_swap(&m_state, 0, 1)) != 0) {
+        if (lock != 2)
+            lock = __atomic_exchange_n(&m_state, 2, __ATOMIC_ACQ_REL);
+        while (lock) {
+            j6_futex_wait(&m_state, 2, 0);
+            lock = __atomic_exchange_n(&m_state, 2, __ATOMIC_ACQ_REL);
+        }
+    }
+}
+
+void
+mutex::unlock()
+{
+    if (__atomic_fetch_sub(&m_state, 1, __ATOMIC_ACQ_REL) != 1) {
+        __atomic_store_n(&m_state, 0, __ATOMIC_RELEASE);
+        j6_futex_wake(&m_state, 1);
+    }
+}
+
+
+} // namespace j6
+
+#endif // __j6kernel