Corinna Vinschen corinna-cygwin@cygwin.com
Wed Jul 1 10:47:00 GMT 2015

On Jun 30 16:13, Ken Brown wrote:
> On 6/30/2015 3:55 PM, Corinna Vinschen wrote:
> >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.
> >>>
> >>>Hi Corinna and Ben,
> >>>
> >>>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 following in
> >>>the emacs *scratch* buffer:
> >>>
> >>>(setq max-specpdl-size 83200000
> >>>       max-lisp-eval-depth 640000)
> >>>(defun foo () (foo))
> >>>(foo)
> >>>
> >>>(The 'setq' is to override emacs's built-in protection against too-deeply
> >>>nested lisp function calls.)
> >>>
> >>>On the other hand, emacs doesn't really make a full recovery.  For example,
> >>>if I try to call a subprocess (e.g., 'C-x d' to list a directory), I get a
> >>>fork error:
> >>>
> >>>Debugger entered--Lisp error: (file-error "Doing vfork" "Resource
> >>>temporarily unavailable")
> >>
> >>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
> >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.
> Thanks for the info, that's good to know.  Just out of curiosity, were you
> able to modify your testcase for this, or did you test with emacs?

I just added a fork call to my testcase right after the last printf.


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