On this machine, a single core Athlon 64 with a 32 bit current Linus' 
git head kernel, I get a lock-up early in the boot process. (A dmesg 
output of a successful boot-up of kernel 4.1.0 up to and slightly passed 
the point where the git head kernel locks up is attached).

A photo of the lock-up appears at:

http://www.users.on.net/~arthur.marsh/20150706462.jpg


git-bisect reveals the bad commit as:


  git bisect good
93c2e105f6bcee231c951ba0e56e84505c4b0483 is the first bad commit
commit 93c2e105f6bcee231c951ba0e56e84505c4b0483
Author: Peter Zijlstra <peterz@infradead.org>
Date:   Wed May 27 11:09:37 2015 +0930

     module: Optimize __module_address() using a latched RB-tree

     Currently __module_address() is using a linear search through all
     modules in order to find the module corresponding to the provided
     address. With a lot of modules this can take a lot of time.

     One of the users of this is kernel_text_address() which is employed
     in many stack unwinders; which in turn are used by perf-callchain and
     ftrace (possibly from NMI context).

     So by optimizing __module_address() we optimize many stack unwinders
     which are used by both perf and tracing in performance sensitive code.

     Cc: Rusty Russell <rusty@rustcorp.com.au>
     Cc: Steven Rostedt <rostedt@goodmis.org>
     Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
     Cc: Oleg Nesterov <oleg@redhat.com>
     Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
     Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
     Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

:040000 040000 1d6863a47d8521a4edbe44f9e7c3422ad1421d61 
a209090da5e67f56f0997671f5a8b510338511ba M      include
:040000 040000 da680638e9cebc8da54d43cc0e67b77b953430fa 
31063f7ed575d22f0d27b24c76b9a0fb68c7cd40 M      kernel


I am happy to supply further details and run further tests.

Arthur.