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[PATCH RT 19/20] trace: correct off by one while recording the trace-event

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4.1.27-rt31-rc1 stable review patch.
If anyone has any objections, please let me know.

------------------

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Trace events like raw_syscalls show always a preempt code of one. The
reason is that on PREEMPT kernels rcu_read_lock_sched_notrace()
increases the preemption counter and the function recording the counter
is caller within the RCU section.

Cc: stable-rt@vger.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
[ Changed this to upstream version. See commit e947841c0dce ]
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
 kernel/trace/trace_events.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index 64c1ac17af8f..b83d6a4d3912 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -200,6 +200,14 @@ void *ftrace_event_buffer_reserve(struct ftrace_event_buffer *fbuffer,
 
 	local_save_flags(fbuffer->flags);
 	fbuffer->pc = preempt_count();
+	/*
+	 * If CONFIG_PREEMPT is enabled, then the tracepoint itself disables
+	 * preemption (adding one to the preempt_count). Since we are
+	 * interested in the preempt_count at the time the tracepoint was
+	 * hit, we need to subtract one to offset the increment.
+	 */
+	if (IS_ENABLED(CONFIG_PREEMPT))
+		fbuffer->pc--;
 	fbuffer->ftrace_file = ftrace_file;
 
 	fbuffer->event =
-- 
2.8.1

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[PATCH RT 00/20] Linux 4.1.27-rt31-rc1 Steven Rostedt <rostedt@goodmis.org> - 2016-07-12 16:30 +0200
  [PATCH RT 18/20] mm: perform lru_add_drain_all() remotely Steven Rostedt <rostedt@goodmis.org> - 2016-07-12 16:30 +0200
  [PATCH RT 13/20] perf/x86/intel/rapl: Make PMU lock raw Steven Rostedt <rostedt@goodmis.org> - 2016-07-12 16:30 +0200
  [PATCH RT 08/20] net: dev: always take qdiscs busylock in __dev_xmit_skb() Steven Rostedt <rostedt@goodmis.org> - 2016-07-12 16:40 +0200
  [PATCH RT 19/20] trace: correct off by one while recording the trace-event Steven Rostedt <rostedt@goodmis.org> - 2016-07-12 16:40 +0200
  [PATCH RT 09/20] drivers/block/zram: Replace bit spinlocks with rtmutex for -rt Steven Rostedt <rostedt@goodmis.org> - 2016-07-12 16:40 +0200
  [PATCH RT 07/20] mm/zsmalloc: Use get/put_cpu_light in  zs_map_object()/zs_unmap_object() Steven Rostedt <rostedt@goodmis.org> - 2016-07-12 16:40 +0200
  [PATCH RT 15/20] kernel/printk: Dont try to print from IRQ/NMI region Steven Rostedt <rostedt@goodmis.org> - 2016-07-12 16:40 +0200
  [PATCH RT 03/20] ARM: at91: pm: move idle functions to pm.c Steven Rostedt <rostedt@goodmis.org> - 2016-07-12 16:40 +0200
  [PATCH RT 04/20] ARM: at91: remove useless includes and function prototypes Steven Rostedt <rostedt@goodmis.org> - 2016-07-12 16:40 +0200
  [PATCH RT 05/20] usb: gadget: atmel: access the PMC using regmap Steven Rostedt <rostedt@goodmis.org> - 2016-07-12 16:40 +0200
  [PATCH RT 01/20] ARM: at91: pm: simply call at91_pm_init Steven Rostedt <rostedt@goodmis.org> - 2016-07-12 16:40 +0200
  [PATCH RT 10/20] list_bl: fixup bogus lockdep warning Steven Rostedt <rostedt@goodmis.org> - 2016-07-12 16:40 +0200
  Linux 4.1.27-rt31-rc2 Steven Rostedt <rostedt@goodmis.org> - 2016-07-13 01:20 +0200

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