x86: Stop recursive fault in print_context_stack after stack overflow

After printing out the first line of a stack backtrace,
print_context_stack() calls print_ftrace_graph_addr() to check
if it's making a graph of function calls, usually not the case.

But unfortunate ordering of assignments causes this to oops if
an earlier stack overflow corrupted threadinfo->task.  Reorder
to avoid that irritation.

( The fact that there was a stack overflow may often be more
  interesting than the stack that can now be shown; but
  integrating that information with this stacktrace is awkward,
  so leave it to overflow reporting. )

Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Link: http://lkml.kernel.org/r/20120323225648.15DD5A033B@akpm.mtv.corp.google.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/dumpstack.c

@@ -37,13 +37,16 @@ print_ftrace_graph_addr(unsigned long addr, void *data,
 			const struct stacktrace_ops *ops,
 			struct thread_info *tinfo, int *graph)
 {
-	struct task_struct *task = tinfo->task;
+	struct task_struct *task;
 	unsigned long ret_addr;
-	int index = task->curr_ret_stack;
+	int index;
 
 	if (addr != (unsigned long)return_to_handler)
 		return;
 
+	task = tinfo->task;
+	index = task->curr_ret_stack;
+
 	if (!task->ret_stack || index < *graph)
 		return;