Patch "tracing: Have preempt(irqs)off trace preempt disabled functions" has been added to the 4.5-stable tree

This is a note to let you know that I've just added the patch titled

tracing: Have preempt(irqs)off trace preempt disabled functions

to the 4.5-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
tracing-have-preempt-irqs-off-trace-preempt-disabled-functions.patch
and it can be found in the queue-4.5 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: "Steven Rostedt (Red Hat)" <[email protected]>
Date: Fri, 18 Mar 2016 12:27:43 -0400
Subject: tracing: Have preempt(irqs)off trace preempt disabled functions

From: Steven Rostedt (Red Hat) <[email protected]>

commit cb86e05390debcc084cfdb0a71ed4c5dbbec517d upstream.

Joel Fernandes reported that the function tracing of preempt disabled sections was not being reported when running either the preemptirqsoff or preemptoff tracers. This was due to the fact that the function tracer callback for those tracers checked if irqs were disabled before tracing. But this fails when we want to trace preempt off locations as well.

Joel explained that he wanted to see funcitons where interrupts are enabled but preemption was disabled. The expected output he wanted:

   <...>-2265    1d.h1 3419us : preempt_count_sub <-irq_exit
   <...>-2265    1d..1 3419us : __do_softirq <-irq_exit
   <...>-2265    1d..1 3419us : msecs_to_jiffies <-__do_softirq
   <...>-2265    1d..1 3420us : irqtime_account_irq <-__do_softirq
   <...>-2265    1d..1 3420us : __local_bh_disable_ip <-__do_softirq
   <...>-2265    1..s1 3421us : run_timer_softirq <-__do_softirq
   <...>-2265    1..s1 3421us : hrtimer_run_pending <-run_timer_softirq
   <...>-2265    1..s1 3421us : _raw_spin_lock_irq <-run_timer_softirq
   <...>-2265    1d.s1 3422us : preempt_count_add <-_raw_spin_lock_irq
   <...>-2265    1d.s2 3422us : _raw_spin_unlock_irq <-run_timer_softirq
   <...>-2265    1..s2 3422us : preempt_count_sub <-_raw_spin_unlock_irq
   <...>-2265    1..s1 3423us : rcu_bh_qs <-__do_softirq
   <...>-2265    1d.s1 3423us : irqtime_account_irq <-__do_softirq
   <...>-2265    1d.s1 3423us : __local_bh_enable <-__do_softirq

There's a comment saying that the irq disabled check is because there's a possible race that tracing_cpu may be set when the function is executed. But I don't remember that race. For now, I added a check for preemption being enabled too to not record the function, as there would be no race if that was the case. I need to re-investigate this, as I'm now thinking that the tracing_cpu will always be correct. But no harm in keeping the check for now, except for the slight performance hit.

---
 kernel/trace/trace_irqsoff.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

@@ -109,8 +109,12 @@ static int func_prolog_dec(struct trace_
 		return 0;
 
 	local_save_flags(*flags);
-	/* slight chance to get a false positive on tracing_cpu */
-	if (!irqs_disabled_flags(*flags))
+	/*
+	 * Slight chance to get a false positive on tracing_cpu,
+	 * although I'm starting to think there isn't a chance.
+	 * Leave this for now just to be paranoid.
+	 */
+	if (!irqs_disabled_flags(*flags) && !preempt_count())
 		return 0;
 
 	*data = per_cpu_ptr(tr->trace_buffer.data, cpu);

Patches currently in stable-queue which might be from [email protected] are

queue-4.5/tracing-fix-trace_printk-to-print-when-not-using-bprintk.patch queue-4.5/tracing-fix-crash-from-reading-trace_pipe-with-sendfile.patch queue-4.5/tracing-have-preempt-irqs-off-trace-preempt-disabled-functions.patch