[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/kernel/syscalls/futex.cpp

@@ -0,0 +1,84 @@
+#include <j6/errors.h>
+#include <util/node_map.h>
+#include <util/spinlock.h>
+
+#include "clock.h"
+#include "objects/process.h"
+#include "objects/thread.h"
+#include "vm_space.h"
+
+using namespace obj;
+
+namespace syscalls {
+
+struct futex
+{
+    uintptr_t address;
+    wait_queue queue;
+
+    futex() = default;
+    futex(futex &&other) :
+        address {other.address},
+        queue {std::move(other.queue)} {}
+
+    futex & operator=(futex &&other) {
+        address = other.address;
+        queue = std::move(other.queue);
+        return *this;
+    }
+};
+
+inline uint64_t & get_map_key(futex &f) { return f.address; }
+
+util::node_map<uintptr_t, futex> g_futexes;
+util::spinlock g_futexes_lock;
+
+j6_status_t
+futex_wait(const uint32_t *value, uint32_t expected, uint64_t timeout)
+{
+    if (*value != expected)
+        return j6_status_would_block;
+
+    uintptr_t address = reinterpret_cast<uintptr_t>(value);
+    vm_space &space = process::current().space();
+    uintptr_t phys = space.find_physical(address);
+
+    util::scoped_lock lock {g_futexes_lock};
+
+    futex &f = g_futexes[phys];
+    thread& t = thread::current();
+
+    if (timeout) {
+        timeout += clock::get().value();
+        t.set_wake_timeout(timeout);
+    }
+
+    lock.release();
+    f.queue.wait();
+    return j6_status_ok;
+}
+
+j6_status_t
+futex_wake(const uint32_t *value, size_t count)
+{
+    uintptr_t address = reinterpret_cast<uintptr_t>(value);
+    vm_space &space = process::current().space();
+    uintptr_t phys = space.find_physical(address);
+
+    util::scoped_lock lock {g_futexes_lock};
+
+    futex *f = g_futexes.find(phys);
+    if (f) {
+        for (unsigned i = 0; i < count; ++i) {
+            obj::thread *t = f->queue.pop_next();
+            t->wake();
+        }
+
+        if (f->queue.empty())
+            g_futexes.erase(phys);
+    }
+
+    return j6_status_ok;
+}
+
+} // namespace syscalls