Strange gdb backtraces on 64-bit Cygwin

[Ken Brown <kbrown at cornell dot edu>]

There have been many bug reports involving crashes or assertion failures 
in emacs-X11 or emacs-w32 on 64-bit Cygwin.  Many of these reports 
include gdb backtraces that don't make sense.  The one I'm looking at 
right now is emacs bug#17753.  I'll try to make this email 
self-contained, but anyone interested in the context should start at

  http://debbugs.gnu.org/cgi/bugreport.cgi?bug=17753#47

The situation in that bug report is that a gdb backtrace showed a call 
to the emacs function "run_timers" in Thread 2, which shouldn't happen. 
 (It should only be called in the main thread.)  There was speculation 
as to whether this could be the cause of the crash.  An alternative 
theory is that the gdb backtrace is bogus.  I'm pretty sure that the OP 
(Markus) was running gdb-7.6.50-4.  His report was about emacs-X11, but 
I've observed similar backtraces for emacs-w32.

To try to sort this out, I've done the following experiment: I've run 
emacs-w32 under gdb with a breakpoint at run_timers.  I've done this on 
both 32-bit Cygwin and 64-bit Cygwin [*], using both gdb-7.6.50-4 and 
gdb-7.8.  [For the latter I used my own build, since the bugfix we 
discussed in a different thread hasn't yet made it into the Cygwin 
distro.]  Transcripts of the four gdb sessions are attached; the file 
names indicate the gdb version and the platform.

My reading of these transcripts is that gdb-7.6.50-4 on 64-bit Cygwin is 
the outlier, and the strange occurrence of run_timers in Thread 2 is 
therefore likely to be a result of a gdb bug.  But it would be great if 
someone familiar with recent gdb development could shed some light on 
this.  In particular, is it plausible that there was a bug of this 
nature in gdb-7.6 that affected only the 64-bit platform and that has 
since been fixed?

Ken

[*] I was using the 20140905 Cygwin snapshots, but the results were 
similar with cygwin-1.7.32-1.

Attachment: gdb-7.6-x86
Description: Text document

Attachment: gdb-7.6-x86_64
Description: Text document

Attachment: gdb-7.8-x86
Description: Text document

Attachment: gdb-7.8-x86_64
Description: Text document

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