futex: fix example, could go wrong in theory

Start std::memory_order stub...

userland/linux/futex.c

@@ -8,14 +8,25 @@
 
 #include <lkmc/futex.h>
 
-static int futex1, futex2;
+static int sleep_done;
 
 void* main_thread(void *arg) {
     (void)arg;
     puts("child start");
-    lkmc_futex(&futex1, FUTEX_WAKE, 1, NULL, NULL, 0);
-    lkmc_futex(&futex2, FUTEX_WAIT, 0, NULL, NULL, 0);
-    puts("child end");
+    /* We only sleep if sleep_done is 0. In that case, we are certain
+     * that a FUTEX_WAKE is coming later on, so we won't deadlock.
+     *
+     * The while is needed due to the possibility of spurious wakeups.
+     *
+     * The __atomic_load_n is not enough to ensure that we only sleep
+     * if sleep_done is 0, because a sleep_done could happen after the check:
+     * this is guaranteed by the system call itself which atomically checks that
+     * for us as well.
+     */
+    while (!__atomic_load_n(&sleep_done, __ATOMIC_ACQUIRE)) {
+        lkmc_futex(&sleep_done, FUTEX_WAIT, 0, NULL, NULL, 0);
+    }
+    puts("child after parent sleep");
     return NULL;
 }
 
@@ -29,9 +40,19 @@ int main(void) {
         main_thread,
         NULL
     ));
-    lkmc_futex(&futex1, FUTEX_WAIT, 0, NULL, NULL, 0);
     sleep(1);
-    puts("main after sleep");
-    lkmc_futex(&futex2, FUTEX_WAKE, 1, NULL, NULL, 0);
+    puts("parent after sleep");
+    /* Mark the sleep as done. If the child still didn't reach the FUTEX_WAIT for some miracle,
+     * (extremely unlikely because we've just slept for one entire second)
+     * we don't want it to sleep, otherwise the FUTEX_WAKE that we will be done next might happen
+     * before the FUTEX_WAIT, which would have no effect, and so FUTEX_WAIT would sleep forever.
+     *
+     * __ATOMIC_ACQUIRE and __ATOMIC_RELEASE are enough here: together they ensure that once this
+     * store is done, then the load MUST see it, which is exactly what we need to avoid deadlocks.
+     * TODO is RELAXED enough? Why yes/not?
+     * See also: https://cirosantilli.com/linux-kernel-module-cheat#cpp-memory-order
+     */
+    __atomic_store_n(&sleep_done, 1, __ATOMIC_RELEASE);
+    lkmc_futex(&sleep_done, FUTEX_WAKE, 1, NULL, NULL, 0);
     assert(!pthread_join(thread, NULL));
 }