On Fri, Nov 17, 2017 at 1:08 PM Kaushal Modi <kaushal.modi@gmail.com> wrote:
(gdb) xbacktrace
Undefined command: "xbacktrace".  Try "help".
(gdb) 

Looks like that *is* the command.

Then is it possible it's related to https://debbugs.gnu.org/cgi/bugreport.cgi?bug=29332? Let me rebuild with http://git.savannah.gnu.org/cgit/emacs.git/commit/?h=emacs-26&id=648c128b5f5eb8988aabcc2073b706d2561acd15 .. just in case.
 
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Kaushal Modi