kgdbts: (2 of 2) fix single step awareness to work correctly with SMP

kgdbts: (2 of 2) fix single step awareness to work correctly with SMP

The do_fork and sys_open tests have never worked properly on anything
other than a UP configuration with the kgdb test suite.  This is
because the test suite did not fully implement the behavior of a real
debugger.  A real debugger tracks the state of what thread it asked to
single step and can correctly continue other threads of execution or
conditionally stop while waiting for the original thread single step
request to return.

Below is a simple method to cause a fatal kernel oops with the kgdb
test suite on a 2 processor ARM system:

while [ 1 ] ; do ls > /dev/null 2> /dev/null; done&
while [ 1 ] ; do ls > /dev/null 2> /dev/null; done&
echo V1I1F100 > /sys/module/kgdbts/parameters/kgdbts

Very soon after starting the test the kernel will start warning with
messages like:

kgdbts: BP mismatch c002487c expected c0024878
------------[ cut here ]------------
WARNING: at drivers/misc/kgdbts.c:317 check_and_rewind_pc+0x9c/0xc4()
[<c01f6520>] (check_and_rewind_pc+0x9c/0xc4)
[<c01f595c>] (validate_simple_test+0x3c/0xc4)
[<c01f60d4>] (run_simple_test+0x1e8/0x274)

The kernel will eventually recovers, but the test suite has completely
failed to test anything useful.

This patch implements behavior similar to a real debugger that does
not rely on hardware single stepping by using only software planted
breakpoints.

In order to mimic a real debugger, the kgdb test suite now tracks the
most recent thread that was continued (cont_thread_id), with the
intent to single step just this thread.  When the response to the
single step request stops in a different thread that hit the original
break point that thread will now get continued, while the debugger
waits for the thread with the single step pending.  Here is a high
level description of the sequence of events.

   cont_instead_of_sstep = 0;

1) set breakpoint at do_fork
2) continue
3)   Save the thread id where we stop to cont_thread_id
4) Remove breakpoint at do_fork
5) Reset the PC if needed depending on kernel exception type
6) soft single step
7)   Check where we stopped
       if current thread != cont_thread_id {
           if (here for more than 2 times for the same thead) {
              ### must be a really busy system, start test again ###
	      goto step 1
           }
           goto step 5
       } else {
           cont_instead_of_sstep = 0;
       }
8) clean up and run test again if needed
9) Clear out any threads that were waiting on a break point at the
   point in time the test is ended with get_cont_catch().  This
   happens sometimes because breakpoints are used in place of single
   stepping and some threads could have been in the debugger exception
   handling queue because breakpoints were hit concurrently on
   different CPUs.  This also means we wait at least one second before
   unplumbing the debugger connection at the very end, so as respond
   to any debug threads waiting to be serviced.

Cc: stable@vger.kernel.org # >= 3.0
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>