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Date:   Mon, 29 Jul 2019 12:29:48 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Eiichi Tsukata <devel@etsukata.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] tracing: Prevent RCU EQS breakage in preemptirq events
Message-ID: <20190729102948.GY31381@hirez.programming.kicks-ass.net>
References: <20190729010734.3352-1-devel@etsukata.com>
 <CALCETrVavLdQ8Rp+6fmTd7kJJwvRKdaEnudaiMAu8g9ZXuNfWA@mail.gmail.com>
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On Sun, Jul 28, 2019 at 09:25:58PM -0700, Andy Lutomirski wrote:
> On Sun, Jul 28, 2019 at 6:08 PM Eiichi Tsukata <devel@etsukata.com> wrote:

> > diff --git a/kernel/trace/trace_preemptirq.c b/kernel/trace/trace_preemptirq.c
> > index 4d8e99fdbbbe..031b51cb94d0 100644
> > --- a/kernel/trace/trace_preemptirq.c
> > +++ b/kernel/trace/trace_preemptirq.c
> > @@ -10,6 +10,7 @@
> >  #include <linux/module.h>
> >  #include <linux/ftrace.h>
> >  #include <linux/kprobes.h>
> > +#include <linux/context_tracking.h>
> >  #include "trace.h"
> >
> >  #define CREATE_TRACE_POINTS
> > @@ -49,9 +50,14 @@ NOKPROBE_SYMBOL(trace_hardirqs_off);
> >
> >  __visible void trace_hardirqs_on_caller(unsigned long caller_addr)
> >  {
> > +       enum ctx_state prev_state;
> > +
> >         if (this_cpu_read(tracing_irq_cpu)) {
> > -               if (!in_nmi())
> > +               if (!in_nmi()) {
> > +                       prev_state = exception_enter();
> >                         trace_irq_enable_rcuidle(CALLER_ADDR0, caller_addr);
> > +                       exception_exit(prev_state);
> > +               }
> >                 tracer_hardirqs_on(CALLER_ADDR0, caller_addr);
> >                 this_cpu_write(tracing_irq_cpu, 0);
> >         }
> 
> This seems a bit distressing.  Now we're going to do a whole bunch of
> context tracking transitions for each kernel entry.  Would a better
> fix me to change trace_hardirqs_on_caller to skip the trace event if
> the previous state was already IRQs on and, more importantly, to skip
> tracing IRQs off if IRQs were already off?  The x86 code is very
> careful to avoid ever having IRQs on and CONTEXT_USER at the same
> time.

I think they already (try to) do that; see 'tracing_irq_cpu'.