From: Pip Cet <pipcet@gmail.com>
To: Eli Zaretskii <eliz@gnu.org>
Cc: 36609@debbugs.gnu.org, politza@hochschule-trier.de
Subject: bug#36609: 27.0.50; Possible race-condition in threading implementation
Date: Fri, 12 Jul 2019 14:34:44 +0000 [thread overview]
Message-ID: <CAOqdjBdg4Gkz=Zw+EVwTDNaRjLU0nL8FjQB4nsnOkT7=n4AgHw@mail.gmail.com> (raw)
In-Reply-To: <83ftnb2wf9.fsf@gnu.org>
On Fri, Jul 12, 2019 at 1:51 PM Eli Zaretskii <eliz@gnu.org> wrote:
> > From: Pip Cet <pipcet@gmail.com>
> > Date: Fri, 12 Jul 2019 13:40:15 +0000
> > Cc: politza@hochschule-trier.de, 36609@debbugs.gnu.org
> >
> > When a thread is signalled (by thread-signal, which sets another
> > thread's error_symbol) while the signalled thread is inside a
> > select(), thread_select() will return non-locally for that thread, and
> > we fail to release an internal GLib lock through
> > g_main_context_release(). That's the first bug.
>
> We should either release the global lock before the thread exits, or
> defer the acting upon the signal until later. We cannot disable the
> signal handling altogether because it is entirely legitimate to signal
> another thread, and when we do, that other thread will _always_ be
> inside thread_select.
Really? What about thread-yield?
> For the main thread, handling the signal in that situation shouldn't
> be a problem, because it is not going to exit. Right?
I think the main thread can still fail to release the lock...
> > When xg_select() fails to acquire the internal GLib lock, it simply
> > does a select() on the remaining file descriptors:
>
> Why does it fail to acquire that lock?
Because another thread holds it, either an Emacs or a non-Emacs
thread. In both cases, I think we might miss events unless we return
with errno == EINTR.
> > context_acquired = g_main_context_acquire (context)
> > /* FIXME: If we couldn't acquire the context, we just silently proceed
> > because this function handles more than just glib file descriptors.
> > Note that, as implemented, this failure is completely silent: there is
> > no feedback to the caller. */
> >
> > This seems like a second, albeit documented, bug to me. I think we're
> > risking not waking up from the actual select because another
> > (non-Emacs) thread happened to hold the main context at the time.
>
> So what is the proposal for that? spin waiting for the lock?
I don't know, to be honest, but I'm afraid there's currently no better
way. There's a way to take the lock without spinning, but it's been
deprecated.
next prev parent reply other threads:[~2019-07-12 14:34 UTC|newest]
Thread overview: 53+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-11 20:51 bug#36609: 27.0.50; Possible race-condition in threading implementation Andreas Politz
2019-07-12 7:47 ` Eli Zaretskii
2019-07-12 8:05 ` Eli Zaretskii
2019-07-12 9:02 ` Pip Cet
2019-07-12 12:42 ` Eli Zaretskii
2019-07-12 12:57 ` Pip Cet
2019-07-12 13:31 ` Eli Zaretskii
2019-07-12 13:51 ` Pip Cet
2019-07-12 15:05 ` Eli Zaretskii
2019-07-12 18:06 ` Pip Cet
2019-07-12 18:27 ` Eli Zaretskii
2019-07-12 18:34 ` Eli Zaretskii
2019-07-12 19:24 ` Pip Cet
2019-07-12 19:57 ` Eli Zaretskii
2019-07-13 14:37 ` Pip Cet
2019-07-13 15:03 ` Eli Zaretskii
2019-07-13 15:13 ` Eli Zaretskii
2019-07-13 15:54 ` Eli Zaretskii
2019-07-13 15:57 ` Pip Cet
2019-07-13 16:02 ` Eli Zaretskii
2019-07-13 18:17 ` Pip Cet
2020-08-21 12:57 ` Lars Ingebrigtsen
2019-07-13 15:04 ` Andreas Politz
2019-07-12 12:44 ` Pip Cet
2019-07-12 12:55 ` Eli Zaretskii
2019-07-12 13:40 ` Pip Cet
2019-07-12 13:51 ` Eli Zaretskii
2019-07-12 14:34 ` Pip Cet [this message]
2019-07-12 15:02 ` Eli Zaretskii
2019-07-12 19:30 ` Pip Cet
2019-07-13 6:50 ` Eli Zaretskii
2021-06-06 15:50 ` dick.r.chiang
[not found] ` <87fsxv8182.fsf@dick>
2021-06-06 16:35 ` Eli Zaretskii
2021-06-06 19:10 ` dick.r.chiang
2021-06-06 19:27 ` Eli Zaretskii
2021-06-09 21:40 ` dick.r.chiang
2021-06-10 6:46 ` Eli Zaretskii
2021-06-10 11:52 ` dick.r.chiang
2021-06-10 14:18 ` Eli Zaretskii
2021-06-10 14:55 ` dick.r.chiang
2021-06-10 15:04 ` Eli Zaretskii
2021-06-10 21:36 ` dick.r.chiang
2021-06-11 6:00 ` Eli Zaretskii
2021-06-19 17:53 ` Eli Zaretskii
2021-06-19 19:14 ` dick.r.chiang
2021-06-19 19:18 ` Eli Zaretskii
2021-06-19 21:12 ` dick.r.chiang
2021-06-20 11:39 ` Eli Zaretskii
2021-06-20 14:01 ` dick.r.chiang
2021-06-20 15:53 ` Eli Zaretskii
2021-06-25 13:54 ` Eli Zaretskii
2021-06-10 15:35 ` Andreas Schwab
2021-06-10 15:39 ` Eli Zaretskii
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