Re: [PATCH] libstdc++: Implementation of class strand

[Jonathan Wakely <jwakely@redhat.com>]

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On Tue, 23 Mar 2021 at 16:39, Jonathan Wakely <jwakely@redhat.com> wrote:

> On 12/03/21 13:21 +0100, Alessio G. B. via Libstdc++ wrote:
> >I expanded the implementation of the class strand of the Networking
> >TS. Essentially, I
> >implemented a token system so each thread knows when it can execute;
> >the system is organized
> >with 2 integers moving as a clock.
>
> Thanks for this patch. I'm not sure when I'll have time to review it,
> and it might not be in time for the upcoming GCC 11 release. But the
> patch has been received and will get reviewed, thanks.
>

Well I didn't think it would take me two years, sorry about that :-(

+ template<typename _Func>
+ void
+ invoke(unsigned int token, _Func&& __f)
+ {
+  std::unique_lock<std::mutex> __lock(_M_mutex);
+
+  _M_cv.wait(__lock,
+     [token, next_token = _M_next_token]()
+     { return token == next_token; });
+
+  try { decay_t<_Func>{std::forward<_Func>(__f)}(); }
+  catch(...) { }
+
+  _M_next_token++;
+
+  __lock.unlock();
+
+  _M_cv.notify_all();
+ }

It looks like this will run a user-provided function while holding the
mutex lock. Won't that deadlock if the task added to the strand adds
another task to the same strand? Is that forbidden by some requirement in
the TS that I've forgotten?