Change loads of jl_gc_disable_counter to use acquire semantics.

Stores already use seq_cst.

Author: maleadt

src/gc.c

@@ -3497,7 +3497,7 @@ JL_DLLEXPORT void jl_gc_collect(jl_gc_collection_t collection)
 
     jl_task_t *ct = jl_current_task;
     jl_ptls_t ptls = ct->ptls;
-    if (jl_atomic_load_relaxed(&jl_gc_disable_counter)) {
+    if (jl_atomic_load_acquire(&jl_gc_disable_counter)) {
         size_t localbytes = jl_atomic_load_relaxed(&ptls->gc_num.allocd) + gc_num.interval;
         jl_atomic_store_relaxed(&ptls->gc_num.allocd, -(int64_t)gc_num.interval);
         static_assert(sizeof(_Atomic(uint64_t)) == sizeof(gc_num.deferred_alloc), "");
@@ -3548,7 +3548,7 @@ JL_DLLEXPORT void jl_gc_collect(jl_gc_collection_t collection)
     gc_invoke_callbacks(jl_gc_cb_pre_gc_t,
         gc_cblist_pre_gc, (collection));
 
-    if (!jl_atomic_load_relaxed(&jl_gc_disable_counter)) {
+    if (!jl_atomic_load_acquire(&jl_gc_disable_counter)) {
         JL_LOCK_NOGC(&finalizers_lock); // all the other threads are stopped, so this does not make sense, right? otherwise, failing that, this seems like plausibly a deadlock
 #ifndef __clang_gcanalyzer__
         if (_jl_gc_collect(ptls, collection)) {

src/safepoint.c

@@ -127,7 +127,7 @@ int jl_safepoint_start_gc(void)
     // Foreign thread adoption disables the GC and waits for it to finish, however, that may
     // introduce a race between it and this thread checking if the GC is enabled and only
     // then setting jl_gc_running. To avoid that, check again now that we won that race.
-    if (jl_atomic_load_relaxed(&jl_gc_disable_counter)) {
+    if (jl_atomic_load_acquire(&jl_gc_disable_counter)) {
         uv_mutex_unlock(&safepoint_lock);
         return 0;
     }

src/threading.c

@@ -412,11 +412,11 @@ JL_DLLEXPORT jl_gcframe_t **jl_adopt_thread(void) JL_NOTSAFEPOINT_LEAVE
     // we can't use a normal safepoint because we don't have signal handlers yet.
     // we also can't use jl_safepoint_wait_gc because that assumes we're in a task.
     jl_atomic_fetch_add(&jl_gc_disable_counter, 1);
-    while (jl_atomic_load_relaxed(&jl_gc_running) || jl_atomic_load_acquire(&jl_gc_running)) {
+    while (jl_atomic_load_acquire(&jl_gc_running)) {
         jl_cpu_pause();
     }
     // In `jl_safepoint_start_gc` we observe if a foreign thread has asked to disable the GC
-    // Guaranteeing the order of events. 
+    // Guaranteeing the order of events.
 
     // initialize this thread (assign tid, create heap, set up root task)
     jl_ptls_t ptls = jl_init_threadtls(-1);