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From: Eli Zaretskii <eliz@gnu.org>
To: Pip Cet <pipcet@gmail.com>
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 18:02:52 +0300	[thread overview]
Message-ID: <83blxz2t43.fsf@gnu.org> (raw)
In-Reply-To: <CAOqdjBdg4Gkz=Zw+EVwTDNaRjLU0nL8FjQB4nsnOkT7=n4AgHw@mail.gmail.com> (message from Pip Cet on Fri, 12 Jul 2019 14:34:44 +0000)

> From: Pip Cet <pipcet@gmail.com>
> Date: Fri, 12 Jul 2019 14:34:44 +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?

What about it?

You are asking whether, when thread-signal is executed, the thread
which we are signaling is necessarily parked inside thread_select?  If
so, I don't understand your surprise: only one thread can ever be
running, and that is by definition the thread which calls
thread-signal.  All the other threads cannot be running, which means
they are parked either in thread_select or in sys_mutex_lock called
from acquire_global_lock.  Right?

As for thread-yield, I'm not sure I understand how is it related to
the issue we are discussing.

> > 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...

Since the main thread doesn't exit, but just longjmp's to top-level,
and then continues to run, why does it need to release the lock?  Any
thread should hold the lock for as long as it runs.

> > > 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.

OK, and why is it a problem that we continue calling thread_select
regardless of the failure to acquire the Glib lock?  is the problem
this one:

> In both cases, I think we might miss events unless we return
> with errno == EINTR.

?  Or is there something else?  If the problem with missing events,
then which events are those, and what bad things will happen if we
miss them?





  reply	other threads:[~2019-07-12 15:02 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
2019-07-12 15:02             ` Eli Zaretskii [this message]
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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