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Re: Need to block SIGCHLD in libgc and guile internal threads

From: mhw
Subject: Re: Need to block SIGCHLD in libgc and guile internal threads
Date: Fri, 29 Aug 2014 13:36:52 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux)

Doug Evans <address@hidden> writes:

> I think(!) I understand why gdb is hanging when used with libgc 7.4.x.
> This is gdb bug 17247.


> gdb/linux-nat.c calls sigsuspend when the inferior is running and gdb
> needs to wait for it to stop.
> gdb is waiting on a SIGCHLD at this point.
> However, if the SIGCHLD goes to a different thread, say the guile
> finalizer thread or a libgc marker thread then the sigsuspend that gdb
> calls doesn't wake up and gdb is hung.
> So question: Any suggestions for how to approach this?


> I think there's a general issue here that these threads should block
> every signal they're not expecting,

Agreed.  Threads that Guile creates implicitly should block every signal
that they're not expecting.

> diff --git a/libguile/finalizers.c b/libguile/finalizers.c
> index 82f292c..95a022c 100644
> --- a/libguile/finalizers.c
> +++ b/libguile/finalizers.c
> @@ -239,6 +239,12 @@ finalization_thread_proc (void *unused)
>  static void*
>  run_finalization_thread (void *arg)
>  {
> +  {
> +    sigset_t blocked_mask;
> +    sigemptyset (&blocked_mask);
> +    sigaddset (&blocked_mask, SIGCHLD);
> +    sigprocmask (SIG_BLOCK, &blocked_mask, NULL);
> +  }
>    return scm_with_guile (finalization_thread_proc, arg);
>  }

This code should be harmonized with the code at the top of
'signal_delivery_thread' in scmsigs.c:

--8<---------------cut here---------------start------------->8---
static SCM
signal_delivery_thread (void *data)
  int sig;
#if HAVE_PTHREAD_SIGMASK  /* not on mingw, see notes above */
  sigset_t all_sigs;
  sigfillset (&all_sigs);
  /* On libgc 7.1 and earlier, GC_do_blocking doesn't actually do
     anything.  So in that case, libgc will want to suspend the signal
     delivery thread, so we need to allow it to do so by unmasking the
     suspend signal.  */
  sigdelset (&all_sigs, GC_get_suspend_signal ());
  scm_i_pthread_sigmask (SIG_SETMASK, &all_sigs, NULL);
--8<---------------cut here---------------end--------------->8---

On master, where we now require gc-7.2 or later, I guess we should be
able to simplify this and block all signals.

However, it's not clear how to backport this to stable-2.0, which does
not even have a finalization thread, and yet gdb bug 17247 occurs with
Guile 2.0.11.  Any idea how to prevent the problem on stable-2.0?


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