[PATCH] x86: stop recursive fault in print_context_stack after stack overflow

[PATCH] x86: stop recursive fault in print_context_stack after stack overflow

[Hugh Dickins]

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>
---

 arch/x86/kernel/dumpstack.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

--- linux.git/arch/x86/kernel/dumpstack.c	2012-03-18 16:15:34.000000000 -0700
+++ linux/arch/x86/kernel/dumpstack.c	2012-03-23 10:43:28.196084684 -0700
@@ -37,13 +37,16 @@ print_ftrace_graph_addr(unsigned long ad
 			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;