GDB cannot access memory after Emacs abort

GDB cannot access memory after Emacs abort

[Stephen Berman]

I recently experienced an abort in CVS Emacs and was unable to get a
backtrace from GDB.  The Emacs bug causing the abort was fixed, but
Richard Stallman responded to the lack of a backtrace with this comment:

> That could be a serious problem in GDB.  It would be good to talk
> with GDB developers about how to investigate it.  Since the bug's
> cause is known, they could focus on figuring out why GDB fails
> to give a backtrace.

Here is the GDB-relevant part of my bug report about the abort (Emacs
was built using the GTK+ toolkit):

> I attempted to debug in gdb; here's the output when the abort occurs:
>
>   (emacs:19177): Gtk-WARNING **: Can't set a parent on widget which has a parent
>   
>   
>   Gtk-ERROR **: file gtkcontainer.c: line 2641 (gtk_container_propagate_expose): assertion failed: (child->parent == GTK_WIDGET (container))
>   aborting...
>   [Switching to Thread 0xb6ec66c0 (LWP 19177)]
>   
>   Breakpoint 1, abort () at emacs.c:431
>   431       kill (getpid (), SIGABRT);
>
> At this point my desktop (I tried in KDE, GNOME and twm, same behavior
> in all) is totally locked up, but I can switch to a virtual tty and
> there kill emacs with SIGKILL (kill -9); SIGTERM (kill -15) does not do
> the job.  This releases the desktop, but gdb delivers no further
> feedback:
>
>   (gdb) xbacktrace
>   Cannot access memory at address 0x8321b6c
>   (gdb) bt
>   #0  abort () at emacs.c:431
>   Cannot access memory at address 0xbfd6836c
>   Cannot access memory at address 0x8321b6c
>
> Setting a break point at abort() in emacs.c makes no difference.  I
> don't have the GTK+ source code so I cannot debug it there.

Does anyone have an idea why there was no backtrace?

Steve Berman

Re: GDB cannot access memory after Emacs abort

[Michael Snyder]

Hi Stephen, 

See questions inline:

On Sun, 2007-11-11 at 00:42 +0100, Stephen Berman wrote:
> I recently experienced an abort in CVS Emacs and was unable to get a
> backtrace from GDB.  The Emacs bug causing the abort was fixed, but
> Richard Stallman responded to the lack of a backtrace with this comment:
> 
> > That could be a serious problem in GDB.  It would be good to talk
> > with GDB developers about how to investigate it.  Since the bug's
> > cause is known, they could focus on figuring out why GDB fails
> > to give a backtrace.

Out of curiosity, and because RMS seems to think it's
relevant -- what is the bug's cause?

> Here is the GDB-relevant part of my bug report about the abort (Emacs
> was built using the GTK+ toolkit):

What's your host architecture?  OS?  How is gdb configured (host-target
tuple)?

> > I attempted to debug in gdb; here's the output when the abort occurs:
> >
> >   (emacs:19177): Gtk-WARNING **: Can't set a parent on widget which has a parent
> >   
> >   
> >   Gtk-ERROR **: file gtkcontainer.c: line 2641 (gtk_container_propagate_expose): assertion failed: (child->parent == GTK_WIDGET (container))
> >   aborting...
> >   [Switching to Thread 0xb6ec66c0 (LWP 19177)]
> >   
> >   Breakpoint 1, abort () at emacs.c:431
> >   431       kill (getpid (), SIGABRT);
> >
> > At this point my desktop (I tried in KDE, GNOME and twm, same behavior
> > in all) is totally locked up, but I can switch to a virtual tty and
> > there kill emacs with SIGKILL (kill -9); SIGTERM (kill -15) does not do
> > the job. 

Making sure that I understand -- you ran emacs under gdb, 
you set a breakpoint at abort, you hit the breakpoint -- 
and your desktop is locked up?

That seems unusual -- do you have any idea of the cause?

Is it possible that emacs is in an infinite recursion
and has consumed all of virtual memory, or something
of the sort?


>  This releases the desktop, but gdb delivers no further
> > feedback:
> >
> >   (gdb) xbacktrace

OK, I'm not familiar with that command.  "xbacktrace"?

Grepping my gdb source tree for "xbacktrace" produces
no results.

> >   Cannot access memory at address 0x8321b6c

Is that a valid address for your architecture?

> >   (gdb) bt
> >   #0  abort () at emacs.c:431
> >   Cannot access memory at address 0xbfd6836c
> >   Cannot access memory at address 0x8321b6c
> >
> > Setting a break point at abort() in emacs.c makes no difference.  I
> > don't have the GTK+ source code so I cannot debug it there.
> 
> Does anyone have an idea why there was no backtrace?
> 
> Steve Berman
> 

Re: GDB cannot access memory after Emacs abort

[Eli Zaretskii]

> From: Michael Snyder <msnyder@specifix.com>
> Cc: gdb@sources.redhat.com
> Date: Sat, 10 Nov 2007 22:38:14 -0800
> 
> Making sure that I understand -- you ran emacs under gdb, 
> you set a breakpoint at abort, you hit the breakpoint -- 

The .gdbinit file in the Emacs src directory always sets a breakpoint
at `abort', because Emacs calls `abort' when it encounters a ``can't
happen'' situation.

> > >   (gdb) xbacktrace
> 
> OK, I'm not familiar with that command.  "xbacktrace"?

It's defined in Emacs's src/.gdbinint.  I reproduce it below, in case
you are interested, but the upshot of all this is that `bt' doesn't
work, as shown below:

> > >   (gdb) bt
> > >   #0  abort () at emacs.c:431
> > >   Cannot access memory at address 0xbfd6836c
> > >   Cannot access memory at address 0x8321b6c

Stack overflow, maybe?

    define xbacktrace
      set $bt = backtrace_list
      while $bt
	xgettype (*$bt->function)
	if $type == Lisp_Symbol
	  xprintsym (*$bt->function)
	  printf " (0x%x)\n", *$bt->args
	else
	  printf "0x%x ", *$bt->function
	  if $type == Lisp_Vectorlike
	    xgetptr (*$bt->function)
	    set $size = ((struct Lisp_Vector *) $ptr)->size
	    output ($size & PVEC_FLAG) ? (enum pvec_type) ($size & PVEC_TYPE_MASK) : $size & ~gdb_array_mark_flag
	  else
	    printf "Lisp type %d", $type
	  end
	  echo \n
	end
	set $bt = $bt->next
      end
    end
    document xbacktrace
      Print a backtrace of Lisp function calls from backtrace_list.
      Set a breakpoint at Fsignal and call this to see from where
      an error was signaled.
    end

Re: GDB cannot access memory after Emacs abort

[Stephen Berman]

On Sun, 11 Nov 2007 09:44:23 +0200 Eli Zaretskii <eliz@gnu.org> wrote:

>                         the upshot of all this is that `bt' doesn't
> work, as shown below:
>
>> > >   (gdb) bt
>> > >   #0  abort () at emacs.c:431
>> > >   Cannot access memory at address 0xbfd6836c
>> > >   Cannot access memory at address 0x8321b6c
>
> Stack overflow, maybe?

Due to an infinite loop in Emacs?  (I don't know if the bug I reported
caused this, maybe Jan D. can answer that.)  But as I mentioned in my
other followup, I've never experienced an infinite loop in Emacs that
locked up X.  If it was due to a stack overflow, does that mean GDB is
above suspicion in this case?

Steve Berman

Re: GDB cannot access memory after Emacs abort

[Eli Zaretskii]

> From:  Stephen Berman <Stephen.Berman@gmx.net>
> Date:  Mon, 12 Nov 2007 00:01:14 +0100
> 
> >> > >   (gdb) bt
> >> > >   #0  abort () at emacs.c:431
> >> > >   Cannot access memory at address 0xbfd6836c
> >> > >   Cannot access memory at address 0x8321b6c
> >
> > Stack overflow, maybe?
> 
> Due to an infinite loop in Emacs?

Due to infinite recursion, or some complex regexp, who knows?

Perhaps the Linux folks here can suggest how to check for a stack
overflow: it has to do with comparing the value of $sp with some
platform-dependent symbol that holds the top of the stack.

> If it was due to a stack overflow, does that mean GDB is above
> suspicion in this case?

Yes, if we prove that it's a stack overflow.

Re: GDB cannot access memory after Emacs abort

[Michael Snyder]

On Mon, 2007-11-12 at 00:01 +0100, Stephen Berman wrote:
> On Sun, 11 Nov 2007 09:44:23 +0200 Eli Zaretskii <eliz@gnu.org> wrote:
> 
> >                         the upshot of all this is that `bt' doesn't
> > work, as shown below:
> >
> >> > >   (gdb) bt
> >> > >   #0  abort () at emacs.c:431
> >> > >   Cannot access memory at address 0xbfd6836c
> >> > >   Cannot access memory at address 0x8321b6c
> >
> > Stack overflow, maybe?
> 
> Due to an infinite loop in Emacs?  (I don't know if the bug I reported
> caused this, maybe Jan D. can answer that.)  But as I mentioned in my
> other followup, I've never experienced an infinite loop in Emacs that
> locked up X.  If it was due to a stack overflow, does that mean GDB is
> above suspicion in this case?

No, it just means we don't yet have enough information to diagnose it.
Stack overflow could potentially produce a state that was too corrupt
for gdb to decypher, but we don't know yet if that's the case.

Now that I know that the target is x86-linux, I can reasonably 
speculate that 0xbfd6836c looks like a stack address, and 
0x8321b6c looks like a code or data address.  But so far those
are only guesses, and it isn't yet clear why gdb would be unable
to access memory at those addresses.

I wonder -- after the above happens, what do you get if you 
type the following at the (gdb) prompt:

  x /i $eip

If you get the same error (Cannot access memory at ...), 
then perhaps gdb has lost contact with the child process
entirely, and cannot access *any* memory.

If not, then some child memory is accessable and some is not
(which is not entirely surprising) -- and the question becomes,
why is gdb trying to read from memory that is not accessable?

Re: GDB cannot access memory after Emacs abort

[Stephen Berman]

On Sun, 11 Nov 2007 21:15:55 -0800 Michael Snyder <msnyder@specifix.com> wrote:

> On Mon, 2007-11-12 at 00:01 +0100, Stephen Berman wrote:
>> On Sun, 11 Nov 2007 09:44:23 +0200 Eli Zaretskii <eliz@gnu.org> wrote:
>> 
>> >                         the upshot of all this is that `bt' doesn't
>> > work, as shown below:
>> >
>> >> > >   (gdb) bt
>> >> > >   #0  abort () at emacs.c:431
>> >> > >   Cannot access memory at address 0xbfd6836c
>> >> > >   Cannot access memory at address 0x8321b6c
>> >
[...]
> I wonder -- after the above happens, what do you get if you 
> type the following at the (gdb) prompt:
>
>   x /i $eip

After the abort occurs, the desktop locks up, I switch to a virtual tty
and kill -9 the emacs process, releasing the desktop, then type what you
said at the gdb prompt and get this:

0x80f9e56 <abort+6>:    Cannot access memory at address 0x80f9e56

In this case, bt returned this:

Cannot access memory at address 0xbf855e4c
Cannot access memory at address 0x8322c0c

> If you get the same error (Cannot access memory at ...), 
> then perhaps gdb has lost contact with the child process
> entirely, and cannot access *any* memory.

Yes, this is also what Jim Blandy surmised.  But, as I ask in my
response to Blandy, why does the desktop lock up only happen when the
emacs abort is induced while running under gdb?

Steve Berman

Re: GDB cannot access memory after Emacs abort

[Michael Snyder]

On Tue, 2007-11-13 at 23:28 +0100, Stephen Berman wrote:
> On Sun, 11 Nov 2007 21:15:55 -0800 Michael Snyder <msnyder@specifix.com> wrote:
> 
> > On Mon, 2007-11-12 at 00:01 +0100, Stephen Berman wrote:
> >> On Sun, 11 Nov 2007 09:44:23 +0200 Eli Zaretskii <eliz@gnu.org> wrote:
> >> 
> >> >                         the upshot of all this is that `bt' doesn't
> >> > work, as shown below:
> >> >
> >> >> > >   (gdb) bt
> >> >> > >   #0  abort () at emacs.c:431
> >> >> > >   Cannot access memory at address 0xbfd6836c
> >> >> > >   Cannot access memory at address 0x8321b6c
> >> >
> [...]
> > I wonder -- after the above happens, what do you get if you 
> > type the following at the (gdb) prompt:
> >
> >   x /i $eip
> 
> After the abort occurs, the desktop locks up, I switch to a virtual tty
> and kill -9 the emacs process, releasing the desktop, then type what you
> said at the gdb prompt and get this:
> 
> 0x80f9e56 <abort+6>:    Cannot access memory at address 0x80f9e56

Oh yes.  I understand that now, thanks.

What we need, I guess, is to get back into control of 
the gdb without killing the emacs.  Otherwise it is
kind of hard to debug this gdb problem further.


> > If you get the same error (Cannot access memory at ...), 
> > then perhaps gdb has lost contact with the child process
> > entirely, and cannot access *any* memory.
> 
> Yes, this is also what Jim Blandy surmised. 

Well, as I now understand, that's because you killed the emacs.


> But, as I ask in my
> response to Blandy, why does the desktop lock up only happen when the
> emacs abort is induced while running under gdb?

I believe this is now well understood, and not a gdb problem.
In a nutshell, your emacs process has a lock on a shared 
resource (the X keyboard-and-mouse "focus").  It is intended
to keep that lock only briefly, but while in posession of the
lock, it aborts.

Normally an abort would result in the freeing of the lock, 
but since gdb stops the emacs process from exiting, the lock
is not freed, resulting in a deadlock when some other process
(eg. xterm) needs the lock.

This is a problem, but a normal and predictable one.
GDB cannot tell when a debugged process is in posession 
of a lock that will cause other processes to deadlock, 
and it has no way of freeing such locks.

This could happen with any shared, lockable resource.

Re: GDB cannot access memory after Emacs abort

[Dan Nicolaescu]

Michael Snyder <msnyder@specifix.com> writes:

  > On Tue, 2007-11-13 at 23:28 +0100, Stephen Berman wrote:
  > > On Sun, 11 Nov 2007 21:15:55 -0800 Michael Snyder <msnyder@specifix.com> wrote:
  > > 
  > > > On Mon, 2007-11-12 at 00:01 +0100, Stephen Berman wrote:
  > > >> On Sun, 11 Nov 2007 09:44:23 +0200 Eli Zaretskii <eliz@gnu.org> wrote:
  > > >> 
  > > >> >                         the upshot of all this is that `bt' doesn't
  > > >> > work, as shown below:
  > > >> >
  > > >> >> > >   (gdb) bt
  > > >> >> > >   #0  abort () at emacs.c:431
  > > >> >> > >   Cannot access memory at address 0xbfd6836c
  > > >> >> > >   Cannot access memory at address 0x8321b6c
  > > >> >
  > > [...]
  > > > I wonder -- after the above happens, what do you get if you 
  > > > type the following at the (gdb) prompt:
  > > >
  > > >   x /i $eip
  > > 
  > > After the abort occurs, the desktop locks up, I switch to a virtual tty
  > > and kill -9 the emacs process, releasing the desktop, then type what you
  > > said at the gdb prompt and get this:
  > > 
  > > 0x80f9e56 <abort+6>:    Cannot access memory at address 0x80f9e56
  > 
  > Oh yes.  I understand that now, thanks.
  > 
  > What we need, I guess, is to get back into control of 
  > the gdb without killing the emacs.  Otherwise it is
  > kind of hard to debug this gdb problem further.

You can do that by using the power of emacs :-)

Run emacs from CVS
M-x server-start RET
M-x gdb
from this gdb start another emacs session and do whatever you need to
induce the crash (just make sure that in the second instance of emacs
you don't run `server-start')

switch to a console and run 
emacsclient -t 

this should connect to the first emacs instance and give you access to
gdb, you can run all the gdb commands there ... 

Hope this helps.

     

Re: GDB cannot access memory after Emacs abort

[Stephen Berman]

On Tue, 13 Nov 2007 15:26:33 -0800 Dan Nicolaescu <dann@ics.uci.edu> wrote:

> Michael Snyder <msnyder@specifix.com> writes:
[...]
>   > What we need, I guess, is to get back into control of 
>   > the gdb without killing the emacs.  Otherwise it is
>   > kind of hard to debug this gdb problem further.
>
> You can do that by using the power of emacs :-)
>
> Run emacs from CVS
> M-x server-start RET
> M-x gdb
> from this gdb start another emacs session and do whatever you need to
> induce the crash (just make sure that in the second instance of emacs
> you don't run `server-start')
>
> switch to a console and run 
> emacsclient -t 
>
> this should connect to the first emacs instance and give you access to
> gdb, you can run all the gdb commands there ... 
>
> Hope this helps.

Indeed it does, thanks.  (I actually combined your suggestion to use
Emacs's multi-tty capability with Michael Snyder's to attach the emacs
process to gdb: I did the latter from within the Emacs client.  This had
the advantage over running gdb directly from the shell that I could copy
and paste the backtrace from the shell buffer to my followup buffer in
Gnus.)

(Before reading your post I had tried sort of the inverse strategy: I
started emacs under gdb from a virtual console, switched to X, invoked
emacsclient -c from an X terminal and induced the abort.  However, when
I switched back to the virtual console the gdb process was dead.  In
hindsight this was obviously a flawed strategy, but I'm still learning
how to use the multi-tty capability.  When will the doc be ready? ;-) )

Steve Berman

Re: GDB cannot access memory after Emacs abort

[Andreas Schwab]

Stephen Berman <Stephen.Berman@gmx.net> writes:

> After the abort occurs, the desktop locks up, I switch to a virtual tty
> and kill -9 the emacs process, releasing the desktop, then type what you
> said at the gdb prompt and get this:
>
> 0x80f9e56 <abort+6>:    Cannot access memory at address 0x80f9e56

Of course, after forcefully killing the process it does not exist any
more, so there is nothing the debugger can inspect.

I'd suggest running the debugger inside screen, then you can attach to
the screen instance from any other tty and inspect the still running
emacs process.

Andreas.

-- 
Andreas Schwab, SuSE Labs, schwab@suse.de
SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
PGP key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."

Re: GDB cannot access memory after Emacs abort

[Daniel Jacobowitz]

On Sat, Nov 10, 2007 at 10:38:14PM -0800, Michael Snyder wrote:
> > > At this point my desktop (I tried in KDE, GNOME and twm, same behavior
> > > in all) is totally locked up, but I can switch to a virtual tty and
> > > there kill emacs with SIGKILL (kill -9); SIGTERM (kill -15) does not do
> > > the job. 
> 
> Making sure that I understand -- you ran emacs under gdb, 
> you set a breakpoint at abort, you hit the breakpoint -- 
> and your desktop is locked up?
> 
> That seems unusual -- do you have any idea of the cause?

This is pretty common when debugging X programs, IIRC.  I believe
there's some ways in which an application can "own" a display while
something is in progress.

That's just from observation, I don't know much about X programming.

-- 
Daniel Jacobowitz
CodeSourcery

Re: GDB cannot access memory after Emacs abort

[Stephen Berman]

On Sun, 11 Nov 2007 14:22:37 -0500 Daniel Jacobowitz <drow@false.org> wrote:

> On Sat, Nov 10, 2007 at 10:38:14PM -0800, Michael Snyder wrote:
>> > > At this point my desktop (I tried in KDE, GNOME and twm, same behavior
>> > > in all) is totally locked up, but I can switch to a virtual tty and
>> > > there kill emacs with SIGKILL (kill -9); SIGTERM (kill -15) does not do
>> > > the job. 
>> 
>> Making sure that I understand -- you ran emacs under gdb, 
>> you set a breakpoint at abort, you hit the breakpoint -- 
>> and your desktop is locked up?
>> 
>> That seems unusual -- do you have any idea of the cause?
>
> This is pretty common when debugging X programs, IIRC.  I believe
> there's some ways in which an application can "own" a display while
> something is in progress.

That's interesting; do you have any pointers to further information
about this?  Yet, as I mentioned in my other followups, this has never
happened to me before when running Emacs under gdb, even when it's in an
infinite loop.  It sounds like you, too, don't suspect a bug in GDB that
prevented getting a backtrace.

Steve Berman

Re: GDB cannot access memory after Emacs abort

[Daniel Jacobowitz]

On Mon, Nov 12, 2007 at 12:01:33AM +0100, Stephen Berman wrote:
> That's interesting; do you have any pointers to further information
> about this?

Nope, sorry.

-- 
Daniel Jacobowitz
CodeSourcery

Re: GDB cannot access memory after Emacs abort

[Jim Blandy]

Stephen Berman <Stephen.Berman at gmx.net> writes:
> On Sun, 11 Nov 2007 14:22:37 -0500 Daniel Jacobowitz <drow@false.org> wrote:
>
>> On Sat, Nov 10, 2007 at 10:38:14PM -0800, Michael Snyder wrote:
>>> > > At this point my desktop (I tried in KDE, GNOME and twm, same behavior
>>> > > in all) is totally locked up, but I can switch to a virtual tty and
>>> > > there kill emacs with SIGKILL (kill -9); SIGTERM (kill -15) does not do
>>> > > the job. 
>>> 
>>> Making sure that I understand -- you ran emacs under gdb, 
>>> you set a breakpoint at abort, you hit the breakpoint -- 
>>> and your desktop is locked up?
>>> 
>>> That seems unusual -- do you have any idea of the cause?
>>
>> This is pretty common when debugging X programs, IIRC.  I believe
>> there's some ways in which an application can "own" a display while
>> something is in progress.
>
> That's interesting; do you have any pointers to further information
> about this?  Yet, as I mentioned in my other followups, this has never
> happened to me before when running Emacs under gdb, even when it's in an
> infinite loop.  It sounds like you, too, don't suspect a bug in GDB that
> prevented getting a backtrace.

Actually, these are legit X Windows behavior; they're called 'server
grabs'.  They're supposed to be rare (for obvious reasons), but if
Emacs died while it had the server grabbed, you'd certainly not be
able to interact with the debugger in another window.

Am I correct in understanding that:
- your X session locks up, and all your windows are unresponsive, not
  just GDB's and Emacs
- you kill Emacs via some other means, which unfreezes your X session
- now you can interact with GDB again, but GDB can't get the backtrace.

GDB produces backtraces by reading memory from the process.  So if my
sequence above is correct, once you have killed the Emacs process in
another window, then it's expected that GDB won't be able to get its
backtrace.

Re: GDB cannot access memory after Emacs abort

[Andreas Schwab]

Jim Blandy <jimb@codesourcery.com> writes:

> Actually, these are legit X Windows behavior; they're called 'server
> grabs'.  They're supposed to be rare (for obvious reasons), but if
> Emacs died while it had the server grabbed, you'd certainly not be
> able to interact with the debugger in another window.

You can ungrab the server by typing XF86_Ungrab (assigned to C-A-kp-/ by
default).

Andreas.

-- 
Andreas Schwab, SuSE Labs, schwab@suse.de
SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
PGP key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."

Re: GDB cannot access memory after Emacs abort

[Stephen Berman]

On Mon, 12 Nov 2007 20:44:29 +0100 Andreas Schwab <schwab@suse.de> wrote:

> Jim Blandy <jimb@codesourcery.com> writes:
>
>> Actually, these are legit X Windows behavior; they're called 'server
>> grabs'.  They're supposed to be rare (for obvious reasons), but if
>> Emacs died while it had the server grabbed, you'd certainly not be
>> able to interact with the debugger in another window.
>
> You can ungrab the server by typing XF86_Ungrab (assigned to C-A-kp-/ by
> default).

I tried this after the desktop lock-up, but it did not release the
desktop.  I also tried XF86_ClearGrab (C-A-kp-*) but this also had no
effect.  Before these attempts I had confirmed with xev that those were
the correct bindings for these commands.

Steve Berman

Re: GDB cannot access memory after Emacs abort

[Stephen Berman]

On Mon, 12 Nov 2007 09:46:43 -0800 Jim Blandy <jimb@codesourcery.com> wrote:

> Stephen Berman <Stephen.Berman at gmx.net> writes:
>> On Sun, 11 Nov 2007 14:22:37 -0500 Daniel Jacobowitz <drow@false.org> wrote:
>>
>>> On Sat, Nov 10, 2007 at 10:38:14PM -0800, Michael Snyder wrote:
>>>> > > At this point my desktop (I tried in KDE, GNOME and twm, same behavior
>>>> > > in all) is totally locked up, but I can switch to a virtual tty and
>>>> > > there kill emacs with SIGKILL (kill -9); SIGTERM (kill -15) does not do
>>>> > > the job. 
>>>> 
>>>> Making sure that I understand -- you ran emacs under gdb, 
>>>> you set a breakpoint at abort, you hit the breakpoint -- 
>>>> and your desktop is locked up?
>>>> 
>>>> That seems unusual -- do you have any idea of the cause?
>>>
>>> This is pretty common when debugging X programs, IIRC.  I believe
>>> there's some ways in which an application can "own" a display while
>>> something is in progress.
>>
>> That's interesting; do you have any pointers to further information
>> about this?  Yet, as I mentioned in my other followups, this has never
>> happened to me before when running Emacs under gdb, even when it's in an
>> infinite loop.  It sounds like you, too, don't suspect a bug in GDB that
>> prevented getting a backtrace.
>
> Actually, these are legit X Windows behavior; they're called 'server
> grabs'.  They're supposed to be rare (for obvious reasons), but if
> Emacs died while it had the server grabbed, you'd certainly not be
> able to interact with the debugger in another window.

Note, however, that this only happens when running emacs under gdb; if I
start emacs directly from the shell, induce the abort, then the Emacs
window vanishes, but the desktop remains responsive and no other
problems are apparent.

> Am I correct in understanding that:
> - your X session locks up, and all your windows are unresponsive, not
>   just GDB's and Emacs

Yes, all open X apps are unresponsive to keyboard or mouse input, but
the apps continue to operate normally, e.g., the newsticker scrolls, the
displays in gkrellm (time, CPU activity, free memory, etc.) continue to
be updated.

> - you kill Emacs via some other means, which unfreezes your X session

Yes, by switching to a virtual tty.

> - now you can interact with GDB again, but GDB can't get the backtrace.

Yes.

> GDB produces backtraces by reading memory from the process.  So if my
> sequence above is correct, once you have killed the Emacs process in
> another window, then it's expected that GDB won't be able to get its
> backtrace.

So again the (or a) question is, why does this only happen when running
emacs under gdb?

Steve Berman

Re: GDB cannot access memory after Emacs abort

[Michael Snyder]

On Tue, 2007-11-13 at 23:28 +0100, Stephen Berman wrote:
> On Mon, 12 Nov 2007 09:46:43 -0800 Jim Blandy <jimb@codesourcery.com> wrote:
> 
> > Stephen Berman <Stephen.Berman at gmx.net> writes:
> >> On Sun, 11 Nov 2007 14:22:37 -0500 Daniel Jacobowitz <drow@false.org> wrote:
> >>
> >>> On Sat, Nov 10, 2007 at 10:38:14PM -0800, Michael Snyder wrote:
> >>>> > > At this point my desktop (I tried in KDE, GNOME and twm, same behavior
> >>>> > > in all) is totally locked up, but I can switch to a virtual tty and
> >>>> > > there kill emacs with SIGKILL (kill -9); SIGTERM (kill -15) does not do
> >>>> > > the job. 
> >>>> 
> >>>> Making sure that I understand -- you ran emacs under gdb, 
> >>>> you set a breakpoint at abort, you hit the breakpoint -- 
> >>>> and your desktop is locked up?
> >>>> 
> >>>> That seems unusual -- do you have any idea of the cause?
> >>>
> >>> This is pretty common when debugging X programs, IIRC.  I believe
> >>> there's some ways in which an application can "own" a display while
> >>> something is in progress.
> >>
> >> That's interesting; do you have any pointers to further information
> >> about this?  Yet, as I mentioned in my other followups, this has never
> >> happened to me before when running Emacs under gdb, even when it's in an
> >> infinite loop.  It sounds like you, too, don't suspect a bug in GDB that
> >> prevented getting a backtrace.
> >
> > Actually, these are legit X Windows behavior; they're called 'server
> > grabs'.  They're supposed to be rare (for obvious reasons), but if
> > Emacs died while it had the server grabbed, you'd certainly not be
> > able to interact with the debugger in another window.
> 
> Note, however, that this only happens when running emacs under gdb; if I
> start emacs directly from the shell, induce the abort, then the Emacs
> window vanishes, but the desktop remains responsive and no other
> problems are apparent.

Well that's understandable.  emacs has grabbed the focus, presumably
something you are only supposed to do for a brief interval.  If emacs
aborts, it will let go of the focus when it dies, but if gdb stops it
from dying, it can't let go of the focus.

This would happen with any resource that a debugged program 
had a lock on.  You can create deadlocks when you debug things
that have locks on resources.

That's a fact of life.

> > Am I correct in understanding that:
> > - your X session locks up, and all your windows are unresponsive, not
> >   just GDB's and Emacs
> 
> Yes, all open X apps are unresponsive to keyboard or mouse input, but
> the apps continue to operate normally, e.g., the newsticker scrolls, the
> displays in gkrellm (time, CPU activity, free memory, etc.) continue to
> be updated.

I would prefer if we could set this issue aside.
I don't believe it is a gdb issue.  The backtrace
issue is what we should discuss.


> 
> > - you kill Emacs via some other means, which unfreezes your X session
> 
> Yes, by switching to a virtual tty.
> 
> > - now you can interact with GDB again, but GDB can't get the backtrace.
> 
> Yes.
> 
> > GDB produces backtraces by reading memory from the process.  So if my
> > sequence above is correct, once you have killed the Emacs process in
> > another window, then it's expected that GDB won't be able to get its
> > backtrace.
> 
> So again the (or a) question is, why does this only happen when running
> emacs under gdb?

If by "this" you mean the backtrace failure, at this point 
I don't feel we have enough information to answer.  But I 
have not yet read your three other emails.   ;-)

Re: GDB cannot access memory after Emacs abort

[Stephen Berman]

On Tue, 13 Nov 2007 15:05:36 -0800 Michael Snyder <msnyder@specifix.com> wrote:

> On Tue, 2007-11-13 at 23:28 +0100, Stephen Berman wrote:
[...]
>> Note, however, that this only happens when running emacs under gdb; if I
>> start emacs directly from the shell, induce the abort, then the Emacs
>> window vanishes, but the desktop remains responsive and no other
>> problems are apparent.
>
> Well that's understandable.  emacs has grabbed the focus, presumably
> something you are only supposed to do for a brief interval.  If emacs
> aborts, it will let go of the focus when it dies, but if gdb stops it
> from dying, it can't let go of the focus.
>
> This would happen with any resource that a debugged program 
> had a lock on.  You can create deadlocks when you debug things
> that have locks on resources.
>
> That's a fact of life.

Ok, I understand this now.

>> > Am I correct in understanding that:
>> > - your X session locks up, and all your windows are unresponsive, not
>> >   just GDB's and Emacs
>> 
>> Yes, all open X apps are unresponsive to keyboard or mouse input, but
>> the apps continue to operate normally, e.g., the newsticker scrolls, the
>> displays in gkrellm (time, CPU activity, free memory, etc.) continue to
>> be updated.
>
> I would prefer if we could set this issue aside.
> I don't believe it is a gdb issue.  The backtrace
> issue is what we should discuss.

See my followup to your suggestion to attach the emacs process to gdb.

Steve Berman

Re: GDB cannot access memory after Emacs abort

[Andreas Schwab]

Stephen Berman <Stephen.Berman@gmx.net> writes:

> Note, however, that this only happens when running emacs under gdb; if I
> start emacs directly from the shell, induce the abort, then the Emacs
> window vanishes, but the desktop remains responsive and no other
> problems are apparent.

Of course, when an X client disappears any grab held by it is
automatically cancelled.

> So again the (or a) question is, why does this only happen when running
> emacs under gdb?

Because the process is _stopped_, not killed.

Andreas.

-- 
Andreas Schwab, SuSE Labs, schwab@suse.de
SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
PGP key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."

Re: GDB cannot access memory after Emacs abort

[Vladimir Prus]

Daniel Jacobowitz wrote:

> On Sat, Nov 10, 2007 at 10:38:14PM -0800, Michael Snyder wrote:
>> 
>> Making sure that I understand -- you ran emacs under gdb,
>> you set a breakpoint at abort, you hit the breakpoint --
>> and your desktop is locked up?
>> 
>> That seems unusual -- do you have any idea of the cause?
> 
> This is pretty common when debugging X programs, IIRC.  I believe
> there's some ways in which an application can "own" a display while
> something is in progress.

I believe it's XGrabPointer, which causes all mouse and/or keyboard
event to be delivered to specific application. If that application is
stopped by gdb, it means, in effect, that events are not processed at all.

I have no idea if that's the cause, since the emacs diff posted in another
email does not have any apparent locking thing, but it very likely.

>> > > At this point my desktop (I tried in KDE, GNOME and twm, same
>> > > behavior in all)

I believe this is dependent on what toolkit the application being debugged
uses, not what other applications are running.

- Volodya

Re: GDB cannot access memory after Emacs abort

[Stephen Berman]

On Mon, 12 Nov 2007 10:38:36 +0300 Vladimir Prus <ghost@cs.msu.su> wrote:

> Daniel Jacobowitz wrote:
>
>> On Sat, Nov 10, 2007 at 10:38:14PM -0800, Michael Snyder wrote:
>>> 
>>> Making sure that I understand -- you ran emacs under gdb,
>>> you set a breakpoint at abort, you hit the breakpoint --
>>> and your desktop is locked up?
>>> 
>>> That seems unusual -- do you have any idea of the cause?
>> 
>> This is pretty common when debugging X programs, IIRC.  I believe
>> there's some ways in which an application can "own" a display while
>> something is in progress.
>
> I believe it's XGrabPointer, which causes all mouse and/or keyboard
> event to be delivered to specific application. If that application is
> stopped by gdb, it means, in effect, that events are not processed at all.
>
> I have no idea if that's the cause, since the emacs diff posted in another
> email does not have any apparent locking thing, but it very likely.

But as I pointed out in my followup to Jim Blandy, the lockup only
happens when emacs aborts while running under gdb; starting emacs
directly from the shell and inducing the abort does not lock up the
desktop.

Steve Berman

Re: GDB cannot access memory after Emacs abort

[Michael Snyder]

On Tue, 2007-11-13 at 23:29 +0100, Stephen Berman wrote:
> On Mon, 12 Nov 2007 10:38:36 +0300 Vladimir Prus <ghost@cs.msu.su> wrote:
> 
> > Daniel Jacobowitz wrote:
> >
> >> On Sat, Nov 10, 2007 at 10:38:14PM -0800, Michael Snyder wrote:
> >>> 
> >>> Making sure that I understand -- you ran emacs under gdb,
> >>> you set a breakpoint at abort, you hit the breakpoint --
> >>> and your desktop is locked up?
> >>> 
> >>> That seems unusual -- do you have any idea of the cause?
> >> 
> >> This is pretty common when debugging X programs, IIRC.  I believe
> >> there's some ways in which an application can "own" a display while
> >> something is in progress.
> >
> > I believe it's XGrabPointer, which causes all mouse and/or keyboard
> > event to be delivered to specific application. If that application is
> > stopped by gdb, it means, in effect, that events are not processed at all.
> >
> > I have no idea if that's the cause, since the emacs diff posted in another
> > email does not have any apparent locking thing, but it very likely.
> 
> But as I pointed out in my followup to Jim Blandy, the lockup only
> happens when emacs aborts while running under gdb; starting emacs
> directly from the shell and inducing the abort does not lock up the
> desktop.

Hoping you have seen my other replies, I believe we understand
why this happens.

I do have a suggestion.

Run gdb from a non-GUI terminal, eg. a virtual console.

For example, launch emacs normally, WITHOUT gdb, from gnome/kde/gtk/X.
Then go to a virtual console, use 'ps' to determine the process id of
emacs, then start gdb and use the "attach" command to get control of
emacs from gdb.

Now return to the GUI console, and make emacs crash.

When you return to the virtual console, you should be able to debug.

Re: GDB cannot access memory after Emacs abort

[Stephen Berman]

On Tue, 13 Nov 2007 15:14:58 -0800 Michael Snyder <msnyder@specifix.com> wrote:

> On Tue, 2007-11-13 at 23:29 +0100, Stephen Berman wrote:
[...]
>> But as I pointed out in my followup to Jim Blandy, the lockup only
>> happens when emacs aborts while running under gdb; starting emacs
>> directly from the shell and inducing the abort does not lock up the
>> desktop.
>
> Hoping you have seen my other replies, I believe we understand
> why this happens.
>
> I do have a suggestion.
>
> Run gdb from a non-GUI terminal, eg. a virtual console.
>
> For example, launch emacs normally, WITHOUT gdb, from gnome/kde/gtk/X.
> Then go to a virtual console, use 'ps' to determine the process id of
> emacs, then start gdb and use the "attach" command to get control of
> emacs from gdb.
>
> Now return to the GUI console, and make emacs crash.
>
> When you return to the virtual console, you should be able to debug.

Thanks for this suggestion, it worked.  Here's the backtrace:

#0  abort () at emacs.c:431
#1  0xb798526a in g_logv () from /usr/lib/libglib-2.0.so.0
#2  0xb79852a9 in g_log () from /usr/lib/libglib-2.0.so.0
#3  0xb7985320 in g_assert_warning () from /usr/lib/libglib-2.0.so.0
#4  0xb7c7b195 in gtk_container_propagate_expose () from /usr/lib/libgtk-x11-2.0.so.0
#5  0xb7c7b1c1 in ?? () from /usr/lib/libgtk-x11-2.0.so.0
#6  0x085c2d00 in ?? ()
#7  0x086c0a08 in ?? ()
#8  0x087c31f0 in ?? ()
#9  0xb7d23b6a in ?? () from /usr/lib/libgtk-x11-2.0.so.0
#10 0xb7f3aff4 in ?? () from /usr/lib/libgtk-x11-2.0.so.0
#11 0x085c2d00 in ?? ()
#12 0xbfef82a8 in ?? ()
#13 0xb7cf4b42 in ?? () from /usr/lib/libgtk-x11-2.0.so.0
#14 0x086c0a08 in ?? ()
#15 0xbfef82e8 in ?? ()
#16 0xb7c7b1a0 in ?? () from /usr/lib/libgtk-x11-2.0.so.0
#17 0xbfef82e8 in ?? ()
#18 0x0000001e in ?? ()
#19 0x40000036 in ?? ()
#20 0xbfef82b8 in ?? ()
#21 0xb7f3aff4 in ?? () from /usr/lib/libgtk-x11-2.0.so.0
#22 0x085c2d00 in ?? ()
#23 0x085c2d00 in ?? ()
#24 0xbfef82c8 in ?? ()
#25 0xb7c7bbe7 in gtk_container_forall () from /usr/lib/libgtk-x11-2.0.so.0
Backtrace stopped: previous frame inner to this frame (corrupt stack?)

I don't know if this is useful to you or any other gdb hacker.  I don't
have the GTK+ sources installed.  Maybe someone who does can reproduce
the abort and get a more informative backtrace.  In any case, the fact
that I have gotten a backtrace now evidently absolves GDB of the
suspicion that it had a bug preventing a backtrace :-).  And since the
Emacs bug causing the abort was already fixed, and the issue of the
desktop lockup has been explained, I guess we can declare this issue
closed, unless someone thinks the above backtrace is still reason for
concern.

Steve Berman

Re: GDB cannot access memory after Emacs abort

[Michael Snyder]

On Wed, 2007-11-14 at 10:48 +0100, Stephen Berman wrote:

> Thanks for this suggestion, it worked.  Here's the backtrace:

OK, this is great!  See below.

> #0  abort () at emacs.c:431
> #1  0xb798526a in g_logv () from /usr/lib/libglib-2.0.so.0
> #2  0xb79852a9 in g_log () from /usr/lib/libglib-2.0.so.0
> #3  0xb7985320 in g_assert_warning () from /usr/lib/libglib-2.0.so.0
> #4  0xb7c7b195 in gtk_container_propagate_expose () from /usr/lib/libgtk-x11-2.0.so.0
> #5  0xb7c7b1c1 in ?? () from /usr/lib/libgtk-x11-2.0.so.0
> #6  0x085c2d00 in ?? ()
> #7  0x086c0a08 in ?? ()
> #8  0x087c31f0 in ?? ()
[...]

> I don't know if this is useful to you or any other gdb hacker.  I don't
> have the GTK+ sources installed.  Maybe someone who does can reproduce
> the abort and get a more informative backtrace. 

You don't need to have the sources installed, but 
it appears as if GDB can't find symbols for the shared libraries.

Are these libraries installed in an unusual location?
Is LD_LIBRARY_PATH set correctly (in the gdb shell)?
Is there a location (other than /lib, /usr/lib etc)
where you could tell gdb to find the libraries?

See the built in help for "set solib-search-path" 
and "set solib-absolute-prefix".

One more thing that might help is if you can install
the debuggable versions of these libraries (the ones
compiled with -g for debug symbols, and/or the ones
that have not been stripped).


>  In any case, the fact
> that I have gotten a backtrace now evidently absolves GDB of the
> suspicion that it had a bug preventing a backtrace :-).  And since the
> Emacs bug causing the abort was already fixed, and the issue of the
> desktop lockup has been explained, I guess we can declare this issue
> closed, unless someone thinks the above backtrace is still reason for
> concern.

I think we're in accord -- my suggestions were just to
help you if you need to debug further (now or in future).

Michael

Re: GDB cannot access memory after Emacs abort

[Stephen Berman]

On Wed, 14 Nov 2007 03:50:53 -0800 Michael Snyder <msnyder@specifix.com> wrote:

> On Wed, 2007-11-14 at 10:48 +0100, Stephen Berman wrote:
>
>> Thanks for this suggestion, it worked.  Here's the backtrace:
>
> OK, this is great!  See below.
>
>> #0  abort () at emacs.c:431
>> #1  0xb798526a in g_logv () from /usr/lib/libglib-2.0.so.0
>> #2  0xb79852a9 in g_log () from /usr/lib/libglib-2.0.so.0
>> #3  0xb7985320 in g_assert_warning () from /usr/lib/libglib-2.0.so.0
>> #4  0xb7c7b195 in gtk_container_propagate_expose () from /usr/lib/libgtk-x11-2.0.so.0
>> #5  0xb7c7b1c1 in ?? () from /usr/lib/libgtk-x11-2.0.so.0
>> #6  0x085c2d00 in ?? ()
>> #7  0x086c0a08 in ?? ()
>> #8  0x087c31f0 in ?? ()
> [...]
>
>> I don't know if this is useful to you or any other gdb hacker.  I don't
>> have the GTK+ sources installed.  Maybe someone who does can reproduce
>> the abort and get a more informative backtrace. 
>
> You don't need to have the sources installed, but 
> it appears as if GDB can't find symbols for the shared libraries.

What does it mean that in frame #4 of the backtrace the symbol
gtk_container_propagate_expose from /usr/lib/libgtk-x11-2.0.so.0 is
referenced but starting in the next stack frame there is only ?? with
reference to the same library?  That gdb is finding some but not all
symbols?  Note that when I attached the emacs process to gdb, it
returned a slew of message like this:

Reading symbols from /usr/lib/libgtk-x11-2.0.so.0...done.
Loaded symbols for /usr/lib/libgtk-x11-2.0.so.0
Reading symbols from /usr/lib/libgdk-x11-2.0.so.0...done.
Loaded symbols for /usr/lib/libgdk-x11-2.0.so.0
Reading symbols from /usr/lib/libatk-1.0.so.0...done.
Loaded symbols for /usr/lib/libatk-1.0.so.0
Reading symbols from /usr/lib/libgdk_pixbuf-2.0.so.0...done.
...

> Are these libraries installed in an unusual location?

They are all in /usr/lib AFAICT; I didn't install them myself, that's
where the distro I use (openSUSE) put them.

> Is LD_LIBRARY_PATH set correctly (in the gdb shell)?

How do I determine this (show env does not list LD_LIBRARY_PATH)?

> Is there a location (other than /lib, /usr/lib etc)
> where you could tell gdb to find the libraries?

Not that I know of.

Here's an additional datapoint, FWIW: I induced the abort again, and
this time the backtrace was slightly different from the one I posted
before:

#0  abort () at emacs.c:431
#1  0xb790f26a in g_logv () from /usr/lib/libglib-2.0.so.0
#2  0xb790f2a9 in g_log () from /usr/lib/libglib-2.0.so.0
#3  0xb790f320 in g_assert_warning () from /usr/lib/libglib-2.0.so.0
#4  0xb7c05195 in gtk_container_propagate_expose () from /usr/lib/libgtk-x11-2.0.so.0
#5  0xb7c051c1 in ?? () from /usr/lib/libgtk-x11-2.0.so.0
#6  0x0833e1b0 in ceil ()
#7  0x086c35c0 in ?? ()
#8  0x087c59f8 in ?? ()
#9  0xb7cadb6a in ?? () from /usr/lib/libgtk-x11-2.0.so.0
#10 0xb7ec4ff4 in ?? () from /usr/lib/libgtk-x11-2.0.so.0
#11 0x0833e1b0 in ceil ()
#12 0xbfea1228 in ?? ()
#13 0xb7c7eb42 in ?? () from /usr/lib/libgtk-x11-2.0.so.0
#14 0x086c35c0 in ?? ()
#15 0xbfea1268 in ?? ()
#16 0xb7c051a0 in ?? () from /usr/lib/libgtk-x11-2.0.so.0
#17 0xbfea1268 in ?? ()
#18 0x0000001e in ?? ()
#19 0x40000036 in ?? ()
#20 0xbfea1238 in ?? ()
#21 0xb7ec4ff4 in ?? () from /usr/lib/libgtk-x11-2.0.so.0
#22 0x0833e1b0 in ceil ()
#23 0x0833e1b0 in ceil ()
#24 0xbfea1248 in ?? ()
#25 0xb7c05be7 in gtk_container_forall () from /usr/lib/libgtk-x11-2.0.so.0
Backtrace stopped: previous frame inner to this frame (corrupt stack?)

What is the significance of the stack frames
#6  0x0833e1b0 in ceil ()
and so on (note it's always the same address)?

Thanks again for your helpful comments.

Steve Berman

Re: GDB cannot access memory after Emacs abort

[Michael Snyder]

On Wed, 2007-11-14 at 20:22 +0100, Stephen Berman wrote:
> On Wed, 14 Nov 2007 03:50:53 -0800 Michael Snyder <msnyder@specifix.com> wrote:
> 
> > On Wed, 2007-11-14 at 10:48 +0100, Stephen Berman wrote:
> >
> >> Thanks for this suggestion, it worked.  Here's the backtrace:
> >
> > OK, this is great!  See below.
> >
> >> #0  abort () at emacs.c:431
> >> #1  0xb798526a in g_logv () from /usr/lib/libglib-2.0.so.0
> >> #2  0xb79852a9 in g_log () from /usr/lib/libglib-2.0.so.0
> >> #3  0xb7985320 in g_assert_warning () from /usr/lib/libglib-2.0.so.0
> >> #4  0xb7c7b195 in gtk_container_propagate_expose () from /usr/lib/libgtk-x11-2.0.so.0
> >> #5  0xb7c7b1c1 in ?? () from /usr/lib/libgtk-x11-2.0.so.0
> >> #6  0x085c2d00 in ?? ()
> >> #7  0x086c0a08 in ?? ()
> >> #8  0x087c31f0 in ?? ()
> > [...]
> >
> >> I don't know if this is useful to you or any other gdb hacker.  I don't
> >> have the GTK+ sources installed.  Maybe someone who does can reproduce
> >> the abort and get a more informative backtrace. 
> >
> > You don't need to have the sources installed, but 
> > it appears as if GDB can't find symbols for the shared libraries.
> 
> What does it mean that in frame #4 of the backtrace the symbol
> gtk_container_propagate_expose from /usr/lib/libgtk-x11-2.0.so.0 is
> referenced but starting in the next stack frame there is only ?? with
> reference to the same library?  

It means that:
For frame #4, gdb found a PC address that fit inside the bounds
of the text section of libgtk-x1-2.0.so, and corresponded with 
the function "gtk_container_propagate_expose" from that library.

For frame #5, gdb found a PC address that also fit the address
range of that library, but did not correspond with any function
symbol.

For frame #6, gdb found a PC address that did not match with
any known section of an object file (eg. shared library), and
could not be matched against any known function symbol.


> That gdb is finding some but not all
> symbols?  Note that when I attached the emacs process to gdb, it
> returned a slew of message like this:
> 
> Reading symbols from /usr/lib/libgtk-x11-2.0.so.0...done.
> Loaded symbols for /usr/lib/libgtk-x11-2.0.so.0
> Reading symbols from /usr/lib/libgdk-x11-2.0.so.0...done.
> Loaded symbols for /usr/lib/libgdk-x11-2.0.so.0
> Reading symbols from /usr/lib/libatk-1.0.so.0...done.
> Loaded symbols for /usr/lib/libatk-1.0.so.0
> Reading symbols from /usr/lib/libgdk_pixbuf-2.0.so.0...done.

OK -- do you know whether these messages accounted for all 
known libraries used by emacs?  Could there be some libraries
that are missing (therefore gdb does not have symbols for)?

...

> What is the significance of the stack frames
> #6  0x0833e1b0 in ceil ()
> and so on (note it's always the same address)?

It may be garbage.  The stack unwind may have become corrupt
by that point (I can't tell).  If we assume it is meaningful, 
it means that gdb found a PC for the stack frame for which
the nearest corresponding function symbol is "ceil".

Re: GDB cannot access memory after Emacs abort

[Stephen Berman]

On Sat, 10 Nov 2007 22:38:14 -0800 Michael Snyder <msnyder@specifix.com> wrote:

> Hi Stephen, 
>
> See questions inline:
>
> On Sun, 2007-11-11 at 00:42 +0100, Stephen Berman wrote:
>> I recently experienced an abort in CVS Emacs and was unable to get a
>> backtrace from GDB.  The Emacs bug causing the abort was fixed, but
>> Richard Stallman responded to the lack of a backtrace with this comment:
>> 
>> > That could be a serious problem in GDB.  It would be good to talk
>> > with GDB developers about how to investigate it.  Since the bug's
>> > cause is known, they could focus on figuring out why GDB fails
>> > to give a backtrace.
>
> Out of curiosity, and because RMS seems to think it's
> relevant -- what is the bug's cause?

It had to do with the handling of named icons in GTK+ tool bars.  I
don't know the code well enough to say more, but the diff is here:
http://cvs.savannah.gnu.org/viewvc/emacs/emacs/src/gtkutil.c?r1=1.120&r2=1.121&pathrev=MAIN 

>> Here is the GDB-relevant part of my bug report about the abort (Emacs
>> was built using the GTK+ toolkit):
>
> What's your host architecture?  OS?  How is gdb configured (host-target
> tuple)?

Linux escher 2.6.22.12-0.1-default i686 athlon i386 GNU/Linux (openSUSE
10.3)

GNU gdb 6.6.50.20070726-cvs
This GDB was configured as "i586-suse-linux".

> Making sure that I understand -- you ran emacs under gdb, 
> you set a breakpoint at abort, you hit the breakpoint -- 
> and your desktop is locked up?

Yes (as Eli Zaretskii pointed out, Emacs set a breakpoint at abort).

> That seems unusual -- do you have any idea of the cause?

No!  I'm hoping someone here might have some insight.

> Is it possible that emacs is in an infinite recursion
> and has consumed all of virtual memory, or something
> of the sort?

This has happened on (rare) occasion, but it never locked up the
desktop, I could always at least kill -9 the emacs process from within
the X window system (in the case under discussion, I was able to switch
to a virtual tty and kill -9 the emacs process from there, but X was
locked up solid).

>> >   Cannot access memory at address 0x8321b6c
>
> Is that a valid address for your architecture?

How can I determine that?

Anyway, it sound like you don't suspect a bug in GDB that prevented
getting a backtrace, or is that still a possibility?

Steve Berman

Re: GDB cannot access memory after Emacs abort

[Michael Snyder]

On Mon, 2007-11-12 at 00:01 +0100, Stephen Berman wrote:
> On Sat, 10 Nov 2007 22:38:14 -0800 Michael Snyder <msnyder@specifix.com> wrote:
> 
> > Hi Stephen, 
> >
> > See questions inline:
> >
> > On Sun, 2007-11-11 at 00:42 +0100, Stephen Berman wrote:
> >> I recently experienced an abort in CVS Emacs and was unable to get a
> >> backtrace from GDB.  The Emacs bug causing the abort was fixed, but
> >> Richard Stallman responded to the lack of a backtrace with this comment:
> >> 
> >> > That could be a serious problem in GDB.  It would be good to talk
> >> > with GDB developers about how to investigate it.  Since the bug's
> >> > cause is known, they could focus on figuring out why GDB fails
> >> > to give a backtrace.
> >
> > Out of curiosity, and because RMS seems to think it's
> > relevant -- what is the bug's cause?
> 
> It had to do with the handling of named icons in GTK+ tool bars.  I
> don't know the code well enough to say more, but the diff is here:
> http://cvs.savannah.gnu.org/viewvc/emacs/emacs/src/gtkutil.c?r1=1.120&r2=1.121&pathrev=MAIN 

Fair enough -- I think that's out of my depth.  ;-)

> >> Here is the GDB-relevant part of my bug report about the abort (Emacs
> >> was built using the GTK+ toolkit):
> >
> > What's your host architecture?  OS?  How is gdb configured (host-target
> > tuple)?
> 
> Linux escher 2.6.22.12-0.1-default i686 athlon i386 GNU/Linux (openSUSE
> 10.3)
> 
> GNU gdb 6.6.50.20070726-cvs
> This GDB was configured as "i586-suse-linux".

Thanks, that's all very helpful.



> 
> > Making sure that I understand -- you ran emacs under gdb, 
> > you set a breakpoint at abort, you hit the breakpoint -- 
> > and your desktop is locked up?
> 
> Yes (as Eli Zaretskii pointed out, Emacs set a breakpoint at abort).
> 
> > That seems unusual -- do you have any idea of the cause?
> 
> No!  I'm hoping someone here might have some insight.

OK -- just hoping for more info.  

> > Is it possible that emacs is in an infinite recursion
> > and has consumed all of virtual memory, or something
> > of the sort?
> 
> This has happened on (rare) occasion, but it never locked up the
> desktop, I could always at least kill -9 the emacs process from within
> the X window system (in the case under discussion, I was able to switch
> to a virtual tty and kill -9 the emacs process from there, but X was
> locked up solid).

Yeah, that's pretty rare in my experience -- but I'm not
a GUI or X hacker.

Seems like something that shouldn't happen, unless thru a
bug in X.  No client app should be able to take control of
the entire windowing system and prevent anything else from
getting access.

Please note, I'm not at all trying to say "this isn't 
our (gdb's) problem".  

> >> >   Cannot access memory at address 0x8321b6c
> >
> > Is that a valid address for your architecture?
> 
> How can I determine that?

Just thought you might know.  Forget about it.

> Anyway, it sound like you don't suspect a bug in GDB that prevented
> getting a backtrace, or is that still a possibility?

Oh, I didn't mean to imply that at all.
Just asking for more information.

So just to define the problem ...

1) emacs calls abort.  This is apparently due to a bug
   in emacs, for which you already have a patch.

2) While emacs is held at the abort by gdb, your X system
   is frozen and you can't get any other X window client to
   work.  We don't know the cause of this, but it probably
   isn't gdb, so let's forget about it in this context.

3) Within gdb, when you're at the breakpoint at abort, 
   backtrace doesn't work.  This part is within our domain.

Now 3 could well be a bug in gdb, but there are other
possibilities.  Something could have corrupted the stack, 
so badly that gdb can't unwind it.

Personally I don't see how to decide that question, based
on the information we have so far.  Maybe Daniel and/or Eli
might have an idea?

To pursue it further, we can go one of two ways:

A) maybe you can provide us with enough information and
context to reproduce the problem ourselves?  This seems 
unlikely, but maybe, for instance, you know that with 
a certain released version of emacs and a certain released
version of linux, you can give a fixed sequence of commands
and reliably reproduce the crash?

or 

B) we can keep asking you for more information, question
and answer style.

For instance, I'd like to know the output that you get
from the following gdb commands when you're at the breakpoint:

i) info registers
ii) info target
iii) x /64x $esp

Re: GDB cannot access memory after Emacs abort

[Stephen Berman]

On Sun, 11 Nov 2007 21:06:28 -0800 Michael Snyder <msnyder@specifix.com> wrote:
[...]
> To pursue it further, we can go one of two ways:
>
> A) maybe you can provide us with enough information and
> context to reproduce the problem ourselves?  This seems 
> unlikely, but maybe, for instance, you know that with 
> a certain released version of emacs and a certain released
> version of linux, you can give a fixed sequence of commands
> and reliably reproduce the crash?
>
> or 
>
> B) we can keep asking you for more information, question
> and answer style.
>
> For instance, I'd like to know the output that you get
> from the following gdb commands when you're at the breakpoint:
>
> i) info registers
> ii) info target
> iii) x /64x $esp

I posted a followup providing the requested information but it hasn't
yet appeared on the list, perhaps because it was too long (the output of
`info target' was 1815 lines long).  Since I've gotten a backtrace in
the mean time (see my followup to your suggestion to attach the emacs
process to gdb), do you want me to repost or email you this information,
or is it no longer relevant?

Steve Berman

Re: GDB cannot access memory after Emacs abort

[Michael Snyder]

On Wed, 2007-11-14 at 10:50 +0100, Stephen Berman wrote:
> On Sun, 11 Nov 2007 21:06:28 -0800 Michael Snyder <msnyder@specifix.com> wrote:
> [...]
> > To pursue it further, we can go one of two ways:
> >
> > A) maybe you can provide us with enough information and
> > context to reproduce the problem ourselves?  This seems 
> > unlikely, but maybe, for instance, you know that with 
> > a certain released version of emacs and a certain released
> > version of linux, you can give a fixed sequence of commands
> > and reliably reproduce the crash?
> >
> > or 
> >
> > B) we can keep asking you for more information, question
> > and answer style.
> >
> > For instance, I'd like to know the output that you get
> > from the following gdb commands when you're at the breakpoint:
> >
> > i) info registers
> > ii) info target
> > iii) x /64x $esp
> 
> I posted a followup providing the requested information but it hasn't
> yet appeared on the list, perhaps because it was too long (the output of
> `info target' was 1815 lines long).  Since I've gotten a backtrace in
> the mean time (see my followup to your suggestion to attach the emacs
> process to gdb), do you want me to repost or email you this information,
> or is it no longer relevant?

I think it's moot now.
Good luck!