[PATCH 2/2] x86: Make the x86-64 stacktrace code safely callable from scheduler

[Frederic Weisbecker <fweisbec@gmail.com>]

Avoid potential scheduler recursion and deadlock from the
stacktrace code by avoiding rescheduling when we re-enable
preemption.

This robustifies some scheduler trace events like sched switch
when they are used to produce callchains in perf or ftrace.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Steven Rostedt <rostedt@goodmis.org>
---
 arch/x86/kernel/dumpstack_64.c |   14 ++++++++++----
 1 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/dumpstack_64.c b/arch/x86/kernel/dumpstack_64.c
index e71c98d..8e4f1d0 100644
--- a/arch/x86/kernel/dumpstack_64.c
+++ b/arch/x86/kernel/dumpstack_64.c
@@ -143,13 +143,16 @@ void dump_trace(struct task_struct *task, struct pt_regs *regs,
 		unsigned long *stack, unsigned long bp,
 		const struct stacktrace_ops *ops, void *data)
 {
-	const unsigned cpu = get_cpu();
-	unsigned long *irq_stack_end =
-		(unsigned long *)per_cpu(irq_stack_ptr, cpu);
+	unsigned long *irq_stack_end;
 	unsigned used = 0;
 	struct thread_info *tinfo;
 	int graph = 0;
 	unsigned long dummy;
+	int cpu;
+
+	preempt_disable();
+
+	cpu = smp_processor_id();
 
 	if (!task)
 		task = current;
@@ -168,6 +171,7 @@ void dump_trace(struct task_struct *task, struct pt_regs *regs,
 	 * exceptions
 	 */
 	tinfo = task_thread_info(task);
+	irq_stack_end = (unsigned long *)__get_cpu_var(irq_stack_ptr);
 	for (;;) {
 		char *id;
 		unsigned long *estack_end;
@@ -219,7 +223,9 @@ void dump_trace(struct task_struct *task, struct pt_regs *regs,
 	 * This handles the process stack:
 	 */
 	bp = ops->walk_stack(tinfo, stack, bp, ops, data, NULL, &graph);
-	put_cpu();
+
+	/* We want stacktrace to be computable anywhere, even in the scheduler */
+	preempt_enable_no_resched();
 }
 EXPORT_SYMBOL(dump_trace);
 
-- 
1.7.3.2