Use atomic variables for thread communication

Fixes https://github.com/libsdl-org/SDL/issues/10711

(cherry picked from commit a0f36fb85b40338fb24e1f1479bdcc432c06aba6)
(cherry picked from commit a75227aaebc5bc04ad5a9b3225c66db0a707951d)

test/testthread.c

@@ -19,7 +19,7 @@
 #include "SDL.h"
 
 static SDL_TLSID tls;
-static int alive = 0;
+static SDL_atomic_t alive;
 static int testprio = 0;
 
 /* Call this instead of exit(), so we can clean up SDL: atexit() is evil. */
@@ -55,7 +55,7 @@ ThreadFunc(void *data)
     SDL_TLSSet(tls, "baby thread", NULL);
     SDL_Log("Started thread %s: My thread id is %lu, thread data = %s\n",
             (char *)data, SDL_ThreadID(), (const char *)SDL_TLSGet(tls));
-    while (alive) {
+    while (SDL_AtomicGet(&alive)) {
         SDL_Log("Thread '%s' is alive!\n", (char *)data);
 
         if (testprio) {
@@ -76,7 +76,7 @@ killed(int sig)
 {
     SDL_Log("Killed with SIGTERM, waiting 5 seconds to exit\n");
     SDL_Delay(5 * 1000);
-    alive = 0;
+    SDL_AtomicSet(&alive, 0);
     quit(0);
 }
 
@@ -112,7 +112,7 @@ int main(int argc, char *argv[])
     SDL_TLSSet(tls, "main thread", NULL);
     SDL_Log("Main thread data initially: %s\n", (const char *)SDL_TLSGet(tls));
 
-    alive = 1;
+    SDL_AtomicSet(&alive, 1);
     thread = SDL_CreateThread(ThreadFunc, "One", "#1");
     if (!thread) {
         SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "Couldn't create thread: %s\n", SDL_GetError());
@@ -120,12 +120,12 @@ int main(int argc, char *argv[])
     }
     SDL_Delay(5 * 1000);
     SDL_Log("Waiting for thread #1\n");
-    alive = 0;
+    SDL_AtomicSet(&alive, 0);
     SDL_WaitThread(thread, NULL);
 
     SDL_Log("Main thread data finally: %s\n", (const char *)SDL_TLSGet(tls));
 
-    alive = 1;
+    SDL_AtomicSet(&alive, 1);
     (void)signal(SIGTERM, killed);
     thread = SDL_CreateThread(ThreadFunc, "Two", "#2");
     if (!thread) {

test/torturethread.c

@@ -34,8 +34,9 @@ quit(int rc)
 int SDLCALL
 SubThreadFunc(void *data)
 {
-    while (!*(int volatile *)data) {
-        ; /* SDL_Delay(10); */ /* do nothing */
+    SDL_atomic_t *flag = (SDL_atomic_t *)data;
+    while (!SDL_AtomicGet(flag)) {
+        SDL_Delay(10);
     }
     return 0;
 }
@@ -44,7 +45,7 @@ int SDLCALL
 ThreadFunc(void *data)
 {
     SDL_Thread *sub_threads[NUMTHREADS];
-    int flags[NUMTHREADS];
+    SDL_atomic_t flags[NUMTHREADS];
     int i;
     int tid = (int)(uintptr_t)data;
 
@@ -53,7 +54,7 @@ ThreadFunc(void *data)
     for (i = 0; i < NUMTHREADS; i++) {
         char name[64];
         (void)SDL_snprintf(name, sizeof(name), "Child%d_%d", tid, i);
-        flags[i] = 0;
+        SDL_AtomicSet(&flags[i], 0);
         sub_threads[i] = SDL_CreateThread(SubThreadFunc, name, &flags[i]);
     }
 
@@ -64,7 +65,7 @@ ThreadFunc(void *data)
 
     SDL_Log("Thread '%d' sending signals to subthreads\n", tid);
     for (i = 0; i < NUMTHREADS; i++) {
-        flags[i] = 1;
+        SDL_AtomicSet(&flags[i], 1);
         SDL_WaitThread(sub_threads[i], NULL);
     }