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Message-ID:	<5205B364.8090007@cs.utoronto.ca>
Date:	Fri, 09 Aug 2013 23:28:36 -0400
From:	Ryan Johnson <ryan DOT johnson AT cs DOT utoronto DOT ca>
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To:	cygwin AT cygwin DOT com
Subject:	Re: 64-bit emacs crashes a lot
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In-Reply-To:	<5203DCCA.1010105@cs.utoronto.ca>

On 08/08/2013 2:00 PM, Ryan Johnson wrote:
> On 08/08/2013 1:42 PM, Ken Brown wrote:
>> On 8/5/2013 11:29 AM, Ryan Johnson wrote:
>>> On 05/08/2013 11:00 AM, Ken Brown wrote:
>>>> On 8/3/2013 3:05 PM, Ryan Johnson wrote:
>>>>> On 02/08/2013 8:07 AM, Ryan Johnson wrote:
>>>>>> On 02/08/2013 7:04 AM, Ken Brown wrote:
>>>>>>> On 8/2/2013 4:02 AM, Corinna Vinschen wrote:
>>>>>>>> On Aug  1 22:46, Ryan Johnson wrote:
>>>>>>>>> Here's a new one... I started a compilation, but before it 
>>>>>>>>> actually
>>>>>>>>> invoked the command it started pegging the CPU. After ^G^G^G, it
>>>>>>>>> crashed with the following:
>>>>>>>>>> Auto-save? (y or n) y
>>>>>>>>>>       0 [main] emacs 5076 C:\cygwin64\bin\emacs-nox.exe: *** 
>>>>>>>>>> fatal
>>>>>>>>>> error - Internal error: TP_NUM_W_BUFS too small 2268032 >= 10.
>>>>>>>>
>>>>>>>> That looks like a memory overwrite.  2268032 is 0x229b80, which 
>>>>>>>> looks
>>>>>>>> suspiciously like a stack address.  And the overwritten value is
>>>>>>>> on the
>>>>>>>> stack, too, well within the cygwin TLS area.  If *this* value gets
>>>>>>>> overwritten, the TLS is probbaly totally hosed at this point. 
>>>>>>>> There's
>>>>>>>> just no way to infer the culprit from this limited info.
>>>>>>>
>>>>>>> Could this be BLODA?  Ryan, I noticed that you wrote in a different
>>>>>>> thread, "I recently migrated to 64-bit cygwin...and so far have not
>>>>>>> had to disable Windows Defender; the latter was a recurring 
>>>>>>> source of
>>>>>>> trouble for my previous 32-bit cygwin install on Win7/64."
>>>>>> This would be a whole new level of nasty from a BLODA... I thought
>>>>>> they only interfered with fork()?
>>>>>>
>>>>>> However, this *is* Windows Defender we're talking about... service
>>>>>> disabled and all cygwin processes restarted. I'll let you know in a
>>>>>> day or so if the crashes go away.
>>>>> Rats. I just had another crash, the "Fatal error 6" variety. Windows
>>>>> Defender has not turned itself back on (it's been known to do 
>>>>> that), and
>>>>> a scan of the BLODA list didn't match anything else on my system.
>>>>>
>>>>> So I don't think it's BLODA...
>>>>>
>>>>> Ideas?
>>>>
>>>> Not really, other than the obvious: (a) Find a reproducible way of
>>>> making emacs-nox crash.  (b) Catch the crash in gdb by setting a
>>>> suitable break point.
Got one! Looks like a stack overflow somewhere in the garbage collector:

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 5316.0x1af4]
0x00000001004df44a in mark_object (arg=<optimized out>)
     at /usr/src/debug/emacs-24.3-4/src/alloc.c:5903
5903            if (CONS_MARKED_P (ptr))
(gdb) bt
#0  0x00000001004df44a in mark_object (arg=<optimized out>)
     at /usr/src/debug/emacs-24.3-4/src/alloc.c:5903
#1  0x00000001004df66e in mark_object (arg=<optimized out>)
     at /usr/src/debug/emacs-24.3-4/src/alloc.c:5914
#2  0x00000001004df593 in mark_object (arg=<optimized out>)
     at /usr/src/debug/emacs-24.3-4/src/alloc.c:5809
#3  0x00000001004df66e in mark_object (arg=<optimized out>)
     at /usr/src/debug/emacs-24.3-4/src/alloc.c:5914
#4  0x00000001004df66e in mark_object (arg=<optimized out>)
     at /usr/src/debug/emacs-24.3-4/src/alloc.c:5914
#5  0x00000001004df585 in mark_object (arg=<optimized out>)
     at /usr/src/debug/emacs-24.3-4/src/alloc.c:5808
#6  0x00000001004dfa4e in mark_vectorlike (
     ptr=0x100f66f28 <bss_sbrk_buffer+6955080>)
     at /usr/src/debug/emacs-24.3-4/src/alloc.c:5501
... snip ...
#2606 0x00000001004dfaf4 in mark_buffer (buffer=<optimized out>)
     at /usr/src/debug/emacs-24.3-4/src/alloc.c:5552
#2607 0x00000001004dff2c in Fgarbage_collect ()
     at /usr/src/debug/emacs-24.3-4/src/alloc.c:5181
#2608 0x0000000000000000 in ?? ()

I have the full backtrace saved to file, let me know if that would be 
useful (there wasn't anything obvious that I could see, just more of the 
same). Meanwhile, I verified that none of the addresses printed is 
repeated, so it doesn't seem to be due to an obvious cycle in the object 
graph.

The crash happened when I foregrounded a stopped emacs. I tried playing 
around with various breakpoints while repeatedly sending ^Z, but no luck 
repeating the "feat" yet.

Ideas?
Ryan

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