linux-kernel - Re: [PATCH] tracing: Prevent RCU EQS breakage in preemptirq events

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Date:   Mon, 29 Jul 2019 21:59:43 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Eiichi Tsukata <devel@...ukata.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tracing: Prevent RCU EQS breakage in preemptirq events

On Tue, 30 Jul 2019 10:50:36 +0900
Eiichi Tsukata <devel@...ukata.com> wrote:

> > 
> > I think they already (try to) do that; see 'tracing_irq_cpu'.
> >   
> 
> Or you mean something like this?
> As for trace_hardirqs_off_caller:

You missed what Peter said.

> 
> diff --git a/kernel/trace/trace_preemptirq.c b/kernel/trace/trace_preemptirq.c
> index 4d8e99fdbbbe..d39478bcf0f2 100644
> --- a/kernel/trace/trace_preemptirq.c
> +++ b/kernel/trace/trace_preemptirq.c
> @@ -66,7 +66,7 @@ __visible void trace_hardirqs_off_caller(unsigned long caller_addr)
>         if (!this_cpu_read(tracing_irq_cpu)) {
                             ^^^^^^^^^^^^^^^
The above makes this called only the first time we disable interrupts.

>                 this_cpu_write(tracing_irq_cpu, 1);
>                 tracer_hardirqs_off(CALLER_ADDR0, caller_addr);
> -               if (!in_nmi())
> +               if (!in_nmi() && !irqs_disabled())

This would always be false. This function is always called with irqs_disabled()!

So no, this is not what is meant.

>                         trace_irq_disable_rcuidle(CALLER_ADDR0, caller_addr);
>         }
> 
> Or
> 
> diff --git a/kernel/trace/trace_preemptirq.c b/kernel/trace/trace_preemptirq.c
> index 4d8e99fdbbbe..e08c5c6ff2b3 100644
> --- a/kernel/trace/trace_preemptirq.c
> +++ b/kernel/trace/trace_preemptirq.c
> @@ -66,8 +66,6 @@ __visible void trace_hardirqs_off_caller(unsigned long caller_addr)
>         if (!this_cpu_read(tracing_irq_cpu)) {
>                 this_cpu_write(tracing_irq_cpu, 1);
>                 tracer_hardirqs_off(CALLER_ADDR0, caller_addr);
> -               if (!in_nmi())
> -                       trace_irq_disable_rcuidle(CALLER_ADDR0, caller_addr);

And this just removes the tracepoint completely?

-- Steve

>         }
> 
> 
> As for trace_hardirqs_on_caller, it is called when IRQs off and CONTEXT_USER.
> So even though we skipped the trace event if the previous state was already IRQs on,
> we will fall into the same situation.