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Date: Tue, 30 Jun 2015 16:13:34 -0400
From: Ken Brown <kbrown AT cornell DOT edu>
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To: cygwin AT cygwin DOT com, Ben Woodard <woodard AT redhat DOT com>
Subject: Re: [ANNOUNCEMENT] TEST RELEASE: Cygwin 2.1.0-0.1
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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?

Ken

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