Re: [musl] [PATCH] mq_notify: fix close/recv race on failure path

[Rich Felker <dalias@libc.org>]

On Sat, Feb 11, 2023 at 09:08:53PM +0300, Alexey Izbyshev wrote:
> On 2023-02-11 20:59, Rich Felker wrote:
> >On Sat, Feb 11, 2023 at 08:50:15PM +0300, Alexey Izbyshev wrote:
> >>On 2023-02-11 20:13, Markus Wichmann wrote:
> >>>On Sat, Feb 11, 2023 at 10:06:03AM -0500, Rich Felker wrote:
> >>>>--- a/src/thread/pthread_detach.c
> >>>>+++ b/src/thread/pthread_detach.c
> >>>>@@ -5,8 +5,12 @@ static int __pthread_detach(pthread_t t)
> >>>> {
> >>>> 	/* If the cas fails, detach state is either already-detached
> >>>> 	 * or exiting/exited, and pthread_join will trap or cleanup. */
> >>>>-	if (a_cas(&t->detach_state, DT_JOINABLE, DT_DETACHED) !=
> >>>>DT_JOINABLE)
> >>>>+	if (a_cas(&t->detach_state, DT_JOINABLE, DT_DETACHED) !=
> >>>>DT_JOINABLE) {
> >>>>+		int cs;
> >>>>+		__pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &cs);
> >>>> 		return __pthread_join(t, 0);
> >>>                ^^^^^^ I think you forgot to rework this.
> >>>>+		__pthread_setcancelstate(cs, 0);
> >>>>+	}
> >>>> 	return 0;
> >>>> }
> >>>>
> >>>
> >>>I see no other obvious missteps, though.
> >>>
> >>Same here, apart from this and misspelled "pthred_detach" in the
> >>commit message, the patches look good to me.
> >>
> >>Regarding the POSIX requirement to run sigev_notify_function in the
> >>context of a detached thread, while it's possible to observe the
> >>wrong detachstate for a short while via pthread_getattr_np after
> >>these patches, I'm not sure there is a standard way to do that. Even
> >>if it exists, this minor issue may be not worth caring about.
> >
> >Would this just be if the notification callback executes before
> >mq_notify returns in the parent?
> 
> Yes, it seems so.
> 
> >I suppose we could have the newly
> >created thread do the work of making the syscall, handling the error
> >case, detaching itself on success and and reporting back to the
> >mq_notify function whether it succeeded or failed via the
> >semaphore/args structure. Thoughts on that?
> >
> Could we just move pthread_detach call to the worker thread to the
> point after pthread_cleanup_pop?

I thought that sounded dubious, in that it might lead to an attempt to
join a detached thread, but maybe it's safe to assume recv will never
return if the mq_notify syscall failed...?

Rich