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X-HELO: calimero.vinschen.de
Date: Tue, 30 Jun 2015 21:55:47 +0200
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: cygwin AT cygwin DOT com, Ben Woodard <woodard AT redhat DOT com>
Subject: Re: [ANNOUNCEMENT] TEST RELEASE: Cygwin 2.1.0-0.1
Message-ID: <20150630195547.GG2918@calimero.vinschen.de>
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On Jun 27 16:52, Corinna Vinschen wrote:
> On Jun 26 18:28, Ken Brown wrote:
> > On 6/26/2015 4:05 PM, Corinna Vinschen wrote:
> > >As for getrlimit(RLIMIT_STACK), I changed that as outlined in my former
> > >mail in git.  On second thought, I also changed the values of
> > >MINSIGSTKSZ and SIGSTKSZ.  Instead of 2K and 8K, they are now defined
> > >as 32K and 64K.  The reason is that we then have enough space on the
> > >alternate stack to install a _cygtls area, should the need arise.
> > >
> > >I created new developer snapshots on https://cygwin.com/snapshots/
> > >Please give them a try.
> > >
> > >Remember to tweak STACK_DANGER_ZONE.  You'll have to rebuild emacs
> > >anyway due to the change to [MIN]SIGSTKSZ.
> >=20
> > Hi Corinna and Ben,
> >=20
> > It works now, in the sense that emacs doesn't crash, and it produces the
> > message "Re-entering top level after C stack overflow".  I tested both
> > 32-bit and 64-bit Cygwin.  My test consisted of evaluating the followin=
g in
> > the emacs *scratch* buffer:
> >=20
> > (setq max-specpdl-size 83200000
> >       max-lisp-eval-depth 640000)
> > (defun foo () (foo))
> > (foo)
> >=20
> > (The 'setq' is to override emacs's built-in protection against too-deep=
ly
> > nested lisp function calls.)
> >=20
> > On the other hand, emacs doesn't really make a full recovery.  For exam=
ple,
> > if I try to call a subprocess (e.g., 'C-x d' to list a directory), I ge=
t a
> > fork error:
> >=20
> > Debugger entered--Lisp error: (file-error "Doing vfork" "Resource
> > temporarily unavailable")
>=20
> The problem is probably that there are still resources in use which
> didn't get free'd.  I'll check next week if I can do anything about it.
> Ideally with a simple testcase than emacs :}

Just FYI, I don't know yet what happens exactly, but this has nothing
to do with the alternate stack.  The child process fails with a status
code 0xC00000FD, STATUS_STACK_OVERFLOW.  Which is kind of weird, given
that the stack overflow has been averted by calling siglongjmp.

I have a hunch.  The stack state in the parent is so that TEB::StackLimit
points into the topmost guard area which, when poked into, triggers the
stack overflow exception.  When forking, Cygwin performs exactly this:
It pokes into the stack to push the guard page out of the way, thus=20
causing the stack memory to be commited, which in turn allows to copy
the stack content from parent to child.

Ok, I'm not sure if I can debug this soon, but at leats it's not
related to sigaltstack handling nor is it a regression.


Corinna

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

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