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Date:	Sun, 26 Oct 2014 07:58:47 -0400
From:	Ken Brown <kbrown AT cornell DOT edu>
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On 10/24/2014 9:52 AM, Corinna Vinschen wrote:
> On Oct 24 14:54, Corinna Vinschen wrote:
>> On Oct 24 12:05, Jon TURNEY wrote:
>>> On 23/10/2014 16:37, Corinna Vinschen wrote:
>>>> On Oct 23 08:04, Ken Brown wrote:
>>>>> Yes, flags register corruption is exactly what Eli suggested in the other
>>>>> bug report I cited.
>>>>
>>>> The aforementioned patch was supposed to fix this problem and it is
>>>> definitely in the current 1.7.32 release...
>>>
>>> I didn't mean to suggest otherwise, just that perhaps a similar problem
>>> exists now.
>>>
>>> So I made the attached test case to explore that.  Maybe I've made an
>>> obvious mistake with it, but on the face of it, it seems to demonstrate
>>> something...
>>>
>>> jon AT tambora /
>>> $ gcc signal-stress.c  -Wall -O0 -g
>>>
>>> jon AT tambora /
>>> $ ./a
>>> failed: 2144210386 isn't equal to 2144210386, apparently
>>
>> So it checks i and j for equality, fails, and then comes up with
>> "42 isn't equal to 42"?  This is weird...
>>
>>> Note there is some odd load dependency. For me, it works fine when it's the
>>> only thing running, but when I start up something CPU intensive, it often
>>> fails...
>>
>> That's... interesting.  I wonder if that only occurs in multi-core or
>> multi-CPU environments.  The fact that i and j are not the same when
>> testing, but then are the same when printf is called looks like a
>> out-of-order execution problem.
>>
>> Is it possible that we have to add CPU memory barriers to the sigdelayed
>> function to avoid stuff like this?
>
> I discussed this with my college Kai Tietz (many thanks to him from
> here), and we came up with a problem in sigdelayed in the 64 bit case:
> pushf is called *after* aligning the stack with andq.  This alignment
> potentially changes the CPU flag values so the restored flags are
> potentially not the flags when entering sigdelayed.
>
> I just applied a patch and created new snapshots on
> https://cygwin.com/snapshots/
>
> I couldn't reprocude the problem locally, so I'd be grateful if you
> could test if that fixes the problem in your testcase, Jon.

I tried Jon's testcase.  With cygwin-1.7.33-0.1, it failed within a few minutes. 
  With cygwin-1.7.33-0.2, I ran it for over an hour with no problem, with the 
system heavily loaded.  So it looks good so far.

> Ken, can you check if this snapshot helps emacs along, too?

The people who have been reporting frequent crashes are aware of the fix.  Now I 
just have to wait and hope I don't hear from them for a few days.

Ken

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