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Date: Fri, 26 Jun 2015 18:28:03 -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
References: <announce DOT 20150620205512 DOT GA28301 AT calimero DOT vinschen DOT de> <558706D5 DOT 1020508 AT cornell DOT edu> <20150622110835 DOT GE28301 AT calimero DOT vinschen DOT de> <20150626111249 DOT GS31223 AT calimero DOT vinschen DOT de> <558D3F4C DOT 6090207 AT cornell DOT edu> <20150626141437 DOT GV31223 AT calimero DOT vinschen DOT de> <558D62D7 DOT 8010709 AT cornell DOT edu> <20150626153647 DOT GX31223 AT calimero DOT vinschen DOT de> <558D8409 DOT 2000400 AT cornell DOT edu> <20150626200512 DOT GA30636 AT calimero DOT vinschen DOT de>
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On 6/26/2015 4:05 PM, Corinna Vinschen wrote:
>
> [CC Ben, please keep him on the CC in replies.  Thank you]
>
>
> Hi Ken,
>
> On Jun 26 12:55, Ken Brown wrote:
>> Hi Corinna,
>>
>> On 6/26/2015 11:36 AM, Corinna Vinschen wrote:
>>> Thanks.  Another question:  How does emacs compute stack_bottom?
>>
>> Very near the beginning of main() it does the following:
>>
>>    char stack_bottom_variable;
>> [...]
>>    /* Record (approximately) where the stack begins.  */
>>    stack_bottom = &stack_bottom_variable;
>
> Thank you.
>
> I created an STC with your code snippets and it now works for me
> (attached for reference).
>
> First problem was the return value of getrlimit(RLIMIT_STACK).
>
> Second problem is emacs.  The check for an offset of the offending
> address in si_addr being less than 16K (STACK_DANGER_ZONE) is
> non-portable, putting it mildly.  This might work on 32 bit Cygwin (I
> didn't test that), but the value is too low for 64 bit Cygwin.  With
> STACK_DANGER_ZONE == 32K the handler works as desired on 64 bit Cygwin.
> Part of the reason is probably the _cygtls area of 12K reserved on each
> thread's stack, which moves the address of &stack_bottom_variable to a
> pretty low value right from the start.  Another the size of the guard
> page area on the main thread (16K).
>
> I had a brief email exchange with a collegue of mine.  Ben allowed me to
> quote him, so here are the important snippets of his replies:
>
> - Rlimits are an old way of doing a job and they were to a certain
>    extent tied up in the pre-thread world of unix processes.  rlimits
>    have never been fully implemented on linux with a way that reproduces
>    the unix way in the pre-thread era. rlimits have become a bit of a
>    historical legacy and are there for posix compliance and code
>    compatibility. The posix language was designed to be vague enough that
>    all implementations could be made to conform.
>
> - Rather than making the system implementation conform to some
>    unspecified behavior, I think it might be a wise idea to fix emacs
>    instead. Looking at the code fragment you posted below(*), I’m not
>    entirely convinced that the code would operate as intended on modern
>    Linux or Unix. Given that, it may be better to make an implementation
>    which does something like the current behavior was intended to do or
>    better yet just remove it as a likely latent bug.
>
> (*) Emacs' handle_sigsegv function.
>
> Of course, for testing purposes this is still nice to have, so thank you
> for this test, I really appreciate it.
>
> 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")
   call-process("ls" nil nil nil "--dired")
   dired-insert-directory("/home/kbrown/src/emacs/32build/" "-al" nil nil t)
   dired-readin-insert()
   dired-readin()
   dired-internal-noselect("~/src/emacs/32build/" nil)
   dired-noselect("~/src/emacs/32build/" nil)
   dired("~/src/emacs/32build/" nil)
   funcall-interactively(dired "~/src/emacs/32build/" nil)
   call-interactively(dired nil nil)
   command-execute(dired)

In view of what Ben said, I don't really care about this from the emacs 
point of view.  I mention it only in case it's useful to you for testing 
the alternate stack.

Ken

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