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Date:	Mon, 6 Jul 2015 18:34:24 +0200
From:	Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To:	cygwin AT cygwin DOT com
Subject:	Re: [ANNOUNCEMENT] TEST RELEASE: Cygwin 2.1.0-0.4
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On Jul  6 11:54, Ken Brown wrote:
> On 7/6/2015 10:45 AM, Corinna Vinschen wrote:
> >Does emacs call setrlimit by any chance?
>=20
> Yes, that's the problem.  The initialization code contains essentially the
> following:
>=20
> if (!getrlimit (RLIMIT_STACK, &rlim))
>     {
>       long newlim;
>       /* Approximate the amount regex.c needs per unit of re_max_failures=
.  */
>       int ratio =3D 20 * sizeof (char *);
>       /* Then add 33% to cover the size of the smaller stacks that regex.c
> 	 successively allocates and discards, on its way to the maximum.  */
>       ratio +=3D ratio / 3;
>       /* Add in some extra to cover
> 	 what we're likely to use for other reasons.  */
>       newlim =3D re_max_failures * ratio + 200000;
>       if (newlim > rlim.rlim_max)
> 	{
> 	  newlim =3D rlim.rlim_max;
> 	  /* Don't let regex.c overflow the stack we have.  */
> 	  re_max_failures =3D (newlim - 200000) / ratio;
> 	}
>       if (rlim.rlim_cur < newlim)
> 	rlim.rlim_cur =3D newlim;
>=20
>       setrlimit (RLIMIT_STACK, &rlim);
>     }

Ok.

> If I disable that code, the problem goes away: rlim_cur is set to the
> expected 0x7fd000 in handle_sigsegv, and emacs recovers from the stack
> overflow.

:)))

> I think I probably should disable that code on Cygwin anyway, because
> there's simply no need for it.  Some time ago I discovered that the defau=
lt
> 2MB stack size was not big enough for emacs on Cygwin, and I made emacs u=
se
> 8MB instead. So there's no need to enlarge it further.

Yes, that probably makes sense.  The computed expression above has
another problem on Windows:  The stacksize is always a multiple of 64K
due to the dreaded allocation granularity.

> >Btw., *if* emacs calls setrlimit and then expects getrlimit to return
> >the *actual* size of the stack, rather than expecting that rlim_cur is
> >just a default value when setting up stacks, it's really doing something
> >borderline.
> >
> >There's simply *no* guarantee that a stack can be extended to this size.
> >Any mmap() call could disallow growing the stack beyond its initial
> >size.  Worse, on Linux you can even mmap so that the stack doesn't
> >grow to the supposed initial maximum size at all.  The reason is that
> >Linux doesn't know the concept of "reserved" virtual memory, but the
> >stack is initially not commited in full either.
> >
> >If you want to know how big your current stack *actually* is, you can
> >utilize pthread_getattr_np on Linux and Cygwin, like this:
> >
> >#include <pthread.h>
> >
> >   static void
> >   handle_sigsegv (int sig, siginfo_t *siginfo, void *arg)
> >   {
> >     pthread_attr_t attr;
> >     size_t stacksize;
> >
> >     if (!pthread_getattr_np (pthread_self (), &attr)
> >	&& !pthread_attr_getstacksize (&attr, &stacksize))
> >       {
> >	beg =3D stack_bottom;
> >	end =3D stack_bottom + stack_direction * stacksize;
> >
> >	[...]
> >
> >Unfortunately this is non-portable as well, as the trailing _np denotes,
> >but at least there *is* a reliable method on Linux and Cygwin...
>=20
> Thanks.  That fixes the problem too, even with the call to setrlimit left
> in. I'll report this to the emacs developers.

Excellent, thanks for testing this!


Corinna

--=20
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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