Merge pull request #1594 from pguyot/w07/fix-timeout-bug

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.

These changes are made under both the "Apache 2.0" and the "GNU Lesser General
Public License 2.1 or later" license terms (dual license).

SPDX-License-Identifier: Apache-2.0 OR LGPL-2.1-or-later

Author: bettio

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)