Fix a concurrency issue yielding a timeout bug

Ensure that `scheduler_make_ready` and `scheduler_set_timeout` calls
`sys_signal`. As `global->waiting_scheduler` was cached or read without
holding `global->schedulers_mutex`, in some cases `sys_signal` was not
called yielding a timeout bug where a message is in the queue but the process
is not awaken.

Signed-off-by: Paul Guyot <pguyot@kallisys.net>

Author: pguyot

src/libAtomVM/scheduler.c

@@ -305,18 +305,16 @@ static void scheduler_make_ready(Context *ctx)
     }
     list_remove(&ctx->processes_list_head);
 #ifndef AVM_NO_SMP
-    bool waiting_scheduler = global->waiting_scheduler;
-    if (!waiting_scheduler) {
+    if (SMP_MUTEX_TRYLOCK(global->schedulers_mutex)) {
         // Start a new scheduler if none are going to take this process.
-        if (SMP_MUTEX_TRYLOCK(global->schedulers_mutex)) {
-            if (global->running_schedulers > 0
-                && global->running_schedulers < global->online_schedulers
-                && !context_get_flags(ctx, Running)) {
-                global->running_schedulers++;
-                smp_scheduler_start(global);
-            }
-            SMP_MUTEX_UNLOCK(global->schedulers_mutex);
+        if (!global->waiting_scheduler
+            && global->running_schedulers > 0
+            && global->running_schedulers < global->online_schedulers
+            && !context_get_flags(ctx, Running)) {
+            global->running_schedulers++;
+            smp_scheduler_start(global);
         }
+        SMP_MUTEX_UNLOCK(global->schedulers_mutex);
     }
 #endif
     // Move to ready queue (from waiting or running)
@@ -326,7 +324,12 @@ static void scheduler_make_ready(Context *ctx)
     list_append(&global->ready_processes, &ctx->processes_list_head);
     SMP_SPINLOCK_UNLOCK(&global->processes_spinlock);
 #ifndef AVM_NO_SMP
-    if (waiting_scheduler) {
+    if (SMP_MUTEX_TRYLOCK(global->schedulers_mutex)) {
+        if (global->waiting_scheduler) {
+            sys_signal(global);
+        }
+        SMP_MUTEX_UNLOCK(global->schedulers_mutex);
+    } else {
         sys_signal(global);
     }
 #elif defined(AVM_TASK_DRIVER_ENABLED)
@@ -410,7 +413,12 @@ void scheduler_set_timeout(Context *ctx, avm_int64_t timeout)
     SMP_SPINLOCK_UNLOCK(&glb->timer_spinlock);
 
 #ifndef AVM_NO_SMP
-    if (glb->waiting_scheduler) {
+    if (SMP_MUTEX_TRYLOCK(glb->schedulers_mutex)) {
+        if (glb->waiting_scheduler) {
+            sys_signal(glb);
+        }
+        SMP_MUTEX_UNLOCK(glb->schedulers_mutex);
+    } else {
         sys_signal(glb);
     }
 #elif defined(AVM_TASK_DRIVER_ENABLED)