Re: [PATCH ftrace/core] tracing: irqsoff: Account for additional preempt_disable

[Masami Hiramatsu]

On Mon, 6 Aug 2018 07:14:35 -0700
Joel Fernandes <joelaf@xxxxxxxxxx> wrote:

> On Mon, Aug 6, 2018 at 7:05 AM, Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
> > On Sun,  5 Aug 2018 20:40:49 -0700
> > "Joel Fernandes (Google)" <joel@xxxxxxxxxxxxxxxxx> wrote:
> >
> >> Recently we tried to make the preemptirqsoff tracer to use irqsoff
> >> tracepoint probes. However this causes issues as reported by Masami:
> >>
> >> [2.271078] Testing tracer preemptirqsoff: .. no entries found ..FAILED!
> >> [2.381015] WARNING: CPU: 0 PID: 1 at /home/mhiramat/ksrc/linux/kernel/
> >> trace/trace.c:1512 run_tracer_selftest+0xf3/0x154
> >>
> >> This is due to the tracepoint code increasing the preempt nesting count
> >> by calling an additional preempt_disable before calling into the
> >> preemptoff tracer which messes up the preempt_count() check in
> >> tracer_hardirqs_off.
> >>
> >> To fix this, make the irqsoff tracer probes balance the additional outer
> >> preempt_disable with a preempt_enable_notrace.
> >
> > I've tested it and ensured this fixes the problem.
> >
> > Tested-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> 
> Thanks!
> 
> >> The other way to fix this is to just use SRCU for all tracepoints.
> >> However we can't do that because we can't use NMIs from RCU context.
> >>
> >> Fixes: c3bc8fd637a9 ("tracing: Centralize preemptirq tracepoints
> >>                       and unify their usage")
> >> Fixes: e6753f23d961 ("tracepoint: Make rcuidle tracepoint callers use SRCU")
> >> Reported-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> >> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> >> ---
> >>  kernel/trace/trace_irqsoff.c | 26 ++++++++++++++++++++++++++
> >>  1 file changed, 26 insertions(+)
> >>
> >> diff --git a/kernel/trace/trace_irqsoff.c b/kernel/trace/trace_irqsoff.c
> >> index 770cd30cda40..ffbf1505d5bc 100644
> >> --- a/kernel/trace/trace_irqsoff.c
> >> +++ b/kernel/trace/trace_irqsoff.c
> >> @@ -603,14 +603,40 @@ static void irqsoff_tracer_stop(struct trace_array *tr)
> >>   */
> >>  static void tracer_hardirqs_on(void *none, unsigned long a0, unsigned long a1)
> >>  {
> >
> > To ensure this function must not be preempted even if we increment preempt
> > count, I think you should check irq_disabled() whole this process, put below
> > here.
> >
> >         if (unlikely(!irq_disabled()))
> >                 return;
> >
> > Since irq_disabled() will be checked in irq_trace() anyway, so no problem
> > to return here when !irq_disabled().
> 
> IRQs can never be enabled here. The trace hooks are called only after
> disabling interrupts, or before enabling them. Right?
> 

Even though, it should be verified or atleast commented on the function header.

Thank you,

-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>