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Re: Now crashing [was Re: guile-2.9.2 and threading

From: Linas Vepstas
Subject: Re: Now crashing [was Re: guile-2.9.2 and threading
Date: Wed, 17 Jul 2019 20:42:13 -0500

Seem to be narrowing it down ... or at least, I have more details ...

On Wed, Jul 17, 2019 at 4:44 PM Linas Vepstas <address@hidden> wrote:

On Wed, Jul 17, 2019 at 12:49 PM Mark H Weaver <address@hidden> wrote:
Hi Linas,

> Investigating the crash with good-old printf's in libguile/vm.c produces
> a vast ocean of prints ... that should have not been printed, and/or should
> have been actual errors, but somehow were not handled by scm_error.
> Using today's git pull of master, here's the diff containing a printf:
> --- a/libguile/vm.c
> +++ b/libguile/vm.c
> @@ -1514,12 +1514,23 @@ thread->guard); fflush(stdout); assert (0); }
>        proc = SCM_SMOB_DESCRIPTOR (proc).apply_trampoline;
>        SCM_FRAME_LOCAL (vp->fp, 0) = proc;
>        return SCM_PROGRAM_CODE (proc);
>      }
> +printf("duuude wrong type to apply!\n"
> +"proc=%lx\n"
> +"ip=%p\n"
> +"sp=%p\n"
> +"fp=%p\n"
> +"sp_min=%p\n"
> +"stack_lim=%p\n",
> +SCM_FRAME_SLOT(vp->fp, 0)->as_u64,
> +vp->ip, vp->sp, vp->fp, vp->sp_min_since_gc, vp->stack_limit);
> +fflush(stdout);
> +
>    vp->ip = SCM_FRAME_VIRTUAL_RETURN_ADDRESS (vp->fp);
>    scm_error (scm_arg_type_key, NULL, "Wrong type to apply: ~S",
>               scm_list_1 (proc), scm_list_1 (proc));
>  }
> As you can see, shortly after my printf, there should have been an
> error report.

Not necessarily.  Note that what 'scm_error' actually does is to raise
an exception.  What happens next depends on what exception handlers are
installed at the time of the error.

OK, but... when I look at what get_callee_vcode() actually does, it seems
to be earnestly trying to fish out the location of a callable function from the
frame pointer, and it does so three plausible ways. If those three don't work
out, then it sets the instruction pointer (to the garbage value), followed by
scm_error(Wrong type to apply). This also looks like an earnest, honest
attempt to report a real error.  But lets double-check.

So who calls get_callee_vcode(), and why, and what did they expect to happen?
Well, that's in three places: one in scm_call_n which is a plausible place where
one might expect the instruction pointer to be set to a valid value. Then there's two
places in vm-engine.c -- "tail-call" and "call" both of which one might plausibly expect
to have a valid instruction pointer.  I can't imagine any valid scenario where anyone
was expecting get_callee_vcode() to actually fail in the normal course of operations. 

There is one more place where  get_callee_vcode() can get called -- via the jump_table,
via a call to scm_jit_enter_mcode()  which issues the code emitted by emit_get_callee_vcode

There are four calls to scm_jit_enter_mcode()  The one that immediately preceeds
the bug is always the one made here, in vm-engine.c:
VM_DEFINE_OP (7, return_values, "return-values", OP1 (X32))  

Right before the call to scm_jit_enter_mcode(), I can printf VP->fp and
and they are... fp=0x7fffe000caf8 fpslot=d33b00 (typical)

the mcode is of course some bytecode that bounces through lightning, and a few insns
later, it arrives at get_callee_vcode() but now  the fp is different, (it changes by 0x20,
always) and the slot is different:  fp=0x7fffe000cad8  and SCM_FRAME_LOCAL(fp,0)
is 0x32 and the 0x32 triggers the scm_error(). (because 0x32 is not any of

(but also, the fpslot=d33b00 is never a SCM_PROGRAM_P or SCM_STRUCTP or
a smob, either... so something got computed along the way ... )

That's what I've got so far. Its highly reproducible.  Quick to happen.  I'm not sure
what to do next. I guess I need to examine emit_get_callee_vcode() and see what
it does, and why.   Any comments, suggestions would be useful.

-- Linas

That is, I can't think of any valid reason why anyone would want to suppress
the scm_error().  And even if I could -- calling scm_error() hundreds of times
per second, as fast as possible, does not seem like efficient coding for dealing
with a call to an invalid address. 

Anyway I'm trying to track down where the invalid value gets set. No luck so far.
There are 6 or 8 places in vm-engine.c where the frame pointer is set to something
that isn't a pointer (which seems like cheating to me: passing non-pointer values
in something called "pointer" is .. well, knee jerk reaction is that it's not wise, but
there may be a deeper reason.)

> There is no error report... until 5-10 minutes later, when the error
> report itself causes a crash.  Before then, I get an endless
> high-speed spew of prints:

It looks like another error is happening within the exception handler.

Well, yes, that also. But given that the instruction pointer contains garbage
its perhaps not entirely surprising... at best, the question is, why didn't it fail

-- Linas


PS: It would be good to pick either 'guile-devel' or 'guile-user' for
    continuation of this thread.  I don't see a reason why it should be
    sent to both lists.

cassette tapes - analog TV - film cameras - you

cassette tapes - analog TV - film cameras - you

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