[tip:x86/urgent] x86: Stop recursive fault in print_context_stack after stack overflow

[tip-bot for Hugh Dickins]

Commit-ID:  65c0ff4079c011232e795e62c74a0a95512b7ac3
Gitweb:     http://git.kernel.org/tip/65c0ff4079c011232e795e62c74a0a95512b7ac3
Author:     Hugh Dickins <hughd@xxxxxxxxxx>
AuthorDate: Fri, 23 Mar 2012 14:02:55 -0700
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Sat, 24 Mar 2012 08:15:04 +0100

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@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Namhyung Kim <namhyung@xxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Link: http://lkml.kernel.org/r/20120323225648.15DD5A033B@xxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 arch/x86/kernel/dumpstack.c |    7 +++++--
 1 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index 4025fe4..90bf130 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/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;
 
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