Don't use dirty backup area anymore

It's to solve race, but magisk will always has a race with init,
so simply don't use it.

Author: yujincheng08

libc/system_properties/system_properties.cpp

@@ -195,9 +195,9 @@ uint32_t SystemProperties::ReadMutablePropertyValue(const prop_info* pi, char* v
     serial = new_serial;
     len = SERIAL_VALUE_LEN(serial);
     if (__predict_false(SERIAL_DIRTY(serial))) {
-      // See the comment in the prop_area constructor.
-      prop_area* pa = contexts_->GetPropAreaForName(pi->name);
-      memcpy(value, pa->dirty_backup_area(), len + 1);
+      __futex_wait(const_cast<atomic_uint_least32_t*>(&pi->serial), serial, nullptr);
+      new_serial = load_const_atomic(&pi->serial, memory_order_relaxed);
+      continue;
     } else {
       memcpy(value, pi->value, len + 1);
     }
@@ -300,17 +300,6 @@ int SystemProperties::Update(prop_info* pi, const char* value, unsigned int len)
   auto* override_pi = const_cast<prop_info*>(have_override ? override_pa->find(pi->name) : nullptr);
 
   uint32_t serial = atomic_load_explicit(&pi->serial, memory_order_relaxed);
-  unsigned int old_len = SERIAL_VALUE_LEN(serial);
-
-  // The contract with readers is that whenever the dirty bit is set, an undamaged copy
-  // of the pre-dirty value is available in the dirty backup area. The fence ensures
-  // that we publish our dirty area update before allowing readers to see a
-  // dirty serial.
-  memcpy(pa->dirty_backup_area(), pi->value, old_len + 1);
-  if (have_override) {
-    memcpy(override_pa->dirty_backup_area(), override_pi->value, old_len + 1);
-  }
-  atomic_thread_fence(memory_order_release);
   serial |= 1;
   atomic_store_explicit(&pi->serial, serial, memory_order_relaxed);
   strncpy(pi->value, value, PROP_VALUE_MAX);