[ANNOUNCEMENT] TEST RELEASE: Cygwin 2.1.0-0.1
Ken Brown
kbrown@cornell.edu
Fri Jun 26 22:28:00 GMT 2015
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(*), Iâm 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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