RFR: 8282475: SafeFetch should not rely on existence of Thread::current [v6]

[Anton Kozlov]

On Wed, 9 Mar 2022 08:35:41 GMT, Johannes Bechberger <duke at openjdk.java.net> wrote:

>> The WXMode for the current thread (on MacOS aarch64) is currently stored in the thread class which is unnecessary as the WXMode is bound to the current OS thread, not the current instance of the thread class.
>> This pull request moves the storage of the current WXMode into a thread local global variable in `os` and changes all related code. SafeFetch depended on the existence of a thread object only because of the WXMode. This pull request therefore removes the dependency, making SafeFetch usable in more contexts.
>
> Johannes Bechberger has updated the pull request incrementally with one additional commit since the last revision:
> 
>   current_thread_wx -> ThreadWX

The change proposes to assume WXWrite as the initial state. Have you considered to extend ThreadWXEnable to fix the assert failure? Something like below (I have not tried to compile though). The refactoring looks OK, but it makes sense to separate it from functional change.


class ThreadWXEnable  {
  Thread* _thread;
  WXMode _old_mode;

public:
  ThreadWXEnable(WXMode new_mode, Thread* thread) :
    _thread(thread)
  {
    if (_thread) {
      _old_mode = _thread->enable_wx(new_mode);
    } else {
      os::current_thread_enable_wx(new_mode);
      _old_mode = WXWrite;
    }
  }
  ~ThreadWXEnable() {
    if (_thread) {
      _thread->enable_wx(_old_mode);
    } else {
      os::current_thread_enable_wx(_old_mode);
    }
  }
};

-------------

PR: https://git.openjdk.java.net/jdk/pull/7727