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Subject[PATCH v3] tracing: Make sure RCU is watching before calling a stack trace
From 8260b0308d3ee0f836cd5ea4b177efeb52ab151f Mon Sep 17 00:00:00 2001
From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
Date: Fri, 12 May 2017 13:15:45 -0400
Subject: [PATCH] tracing: Make sure RCU is watching before calling a stack
trace

As stack tracing now requires "rcu watching", force RCU to be watching when
recording a stack trace.

Link: http://lkml.kernel.org/r/20170512172449.879684501@goodmis.org

Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---

Changes since v2:

Use rcu_is_watching() instead if checking for in_nmi() and irqs
disabled. But now if rcu_is_watching() fails and in_nmi() is true,
bail.

kernel/trace/trace.c | 31 ++++++++++++++++++++++++++++++-
1 file changed, 30 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index fcc9a2d..1122f15 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -2568,7 +2568,36 @@ static inline void ftrace_trace_stack(struct trace_array *tr,
void __trace_stack(struct trace_array *tr, unsigned long flags, int skip,
int pc)
{
- __ftrace_trace_stack(tr->trace_buffer.buffer, flags, skip, pc, NULL);
+ struct ring_buffer *buffer = tr->trace_buffer.buffer;
+
+ if (rcu_is_watching()) {
+ __ftrace_trace_stack(buffer, flags, skip, pc, NULL);
+ return;
+ }
+
+ /*
+ * When an NMI triggers, RCU is enabled via rcu_nmi_enter(),
+ * but if the above rcu_is_watching() failed, then the NMI
+ * triggered someplace critical, and rcu_irq_enter() should
+ * not be called from NMI.
+ */
+ if (unlikely(in_nmi()))
+ return;
+
+ /*
+ * It is possible that a function is being traced in a
+ * location that RCU is not watching. A call to
+ * rcu_irq_enter() will make sure that it is, but there's
+ * a few internal rcu functions that could be traced
+ * where that wont work either. In those cases, we just
+ * do nothing.
+ */
+ if (unlikely(rcu_irq_enter_disabled()))
+ return;
+
+ rcu_irq_enter_irqson();
+ __ftrace_trace_stack(buffer, flags, skip, pc, NULL);
+ rcu_irq_exit_irqson();
}

/**
--
2.9.3
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