[PATCH] tracing/sched: Check preempt_count() for current when reading task->state

[Steven Rostedt <rostedt@goodmis.org>]

When recording the state of a task for the sched_switch tracepoint a check of
task_preempt_count() is performed to see if PREEMPT_ACTIVE is set. This is
because, technically, a task being preempted is really in the TASK_RUNNING
state, and that is what should be recorded when tracing a sched_switch,
even if the task put itself into another state (it hasn't scheduled out
in that state yet).

But with the change to use per_cpu preempt counts, the
task_thread_info(p)->preempt_count is no longer used, and instead
task_preempt_count(p) is used.

The problem is that this does not use the current preempt count but a
stale one from a previous sched_switch. The task_preempt_count(p) uses
saved_preempt_count and not preempt_count(). But for tracing
sched_switch, if p is current, we really want preempt_count().

I hit this bug when I was tracing sleep and the call from do_nanosleep()
scheduled out in the "RUNNING" state.

           sleep-4290  [000] 537272.259992: sched_switch:         sleep:4290 [120] R ==> swapper/0:0 [120]
           sleep-4290  [000] 537272.260015: kernel_stack:         <stack trace>
=> __schedule (ffffffff8150864a)
=> schedule (ffffffff815089f8)
=> do_nanosleep (ffffffff8150b76c)
=> hrtimer_nanosleep (ffffffff8108d66b)
=> SyS_nanosleep (ffffffff8108d750)
=> return_to_handler (ffffffff8150e8e5)
=> tracesys_phase2 (ffffffff8150c844)

After a bit of hair pulling, I found that the state was really
TASK_INTERRUPTIBLE, but the saved_preempt_count had an old PREEMPT_ACTIVE
set and caused the sched_switch tracepoint to show it as RUNNING.

Cc: stable@vger.kernel.org # 3.13+
Fixes: 01028747559a "sched: Create more preempt_count accessors"
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
 include/trace/events/sched.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/include/trace/events/sched.h b/include/trace/events/sched.h
index 0a68d5ae584e..13fbadcc172b 100644
--- a/include/trace/events/sched.h
+++ b/include/trace/events/sched.h
@@ -97,10 +97,14 @@ static inline long __trace_sched_switch_state(struct task_struct *p)
 	long state = p->state;
 
 #ifdef CONFIG_PREEMPT
+	unsigned long pc;
+
+	pc = (p == current) ? preempt_count() : task_preempt_count(p);
+
 	/*
 	 * For all intents and purposes a preempted task is a running task.
 	 */
-	if (task_preempt_count(p) & PREEMPT_ACTIVE)
+	if (pc & PREEMPT_ACTIVE)
 		state = TASK_RUNNING | TASK_STATE_MAX;
 #endif
 
-- 
1.8.1.4