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From:   Andy Lutomirski <luto@kernel.org>
Date:   Mon, 8 Apr 2019 09:08:03 -0700
Message-ID: <CALCETrXbLMirQSXsgbQYfJg0Wr23g+LFoSz3thtttVM1Ms+6Mg@mail.gmail.com>
Subject: Re: [RFC PATCH 0/7] Early task context tracking
To:     Daniel Bristot de Oliveira <bristot@redhat.com>
Cc:     Andy Lutomirski <luto@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>,
        Peter Zijlstra <peterz@infradead.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "Joel Fernandes (Google)" <joel@joelfernandes.org>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Tommaso Cucinotta <tommaso.cucinotta@santannapisa.it>,
        Romulo Silva de Oliveira <romulo.deoliveira@ufsc.br>,
        Clark Williams <williams@redhat.com>, X86 ML <x86@kernel.org>
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On Mon, Apr 8, 2019 at 5:47 AM Daniel Bristot de Oliveira
<bristot@redhat.com> wrote:
>
> On 4/4/19 2:01 AM, Andy Lutomirski wrote:
> >> To resolve this problem, the set/unset of the IRQ/NMI context needs to
> >> be done before the execution of the first C execution, and after its
> >> return. By doing so, and using this method to identify the context in the
> >> trace recursion protection, no more events are lost.
> > I would much rather do the opposite: completely remove context
> > tracking from the asm and, instead, stick it into the C code.  We'd
> > need to make sure that the C code is totally immune from tracing,
> > kprobes, etc, but it would be a nice cleanup.  And then you could fix
> > this bug in C!
> >
> >
>
> Humm... what we could do to have things in C is to set the variable right at the
> begin of the C handler, e.g., do_IRQ(), and right before the return.
>
> But by doing this we would have a problem with two things:
>
> 1) irq handler itself (e.g., do_IRQ())
> 2) functions/tracepoints that might run before and after the handler execution
> (e.g., preemptirq tracer), but still in the IRQ context.
>
> We can work around the first case by checking if (the function is in the
> __irq_entry .text section) in the recursion control.
>
> The second case would still be a problem. For instance, the preemptirq:
> tracepoints in the preemptirq tracer would be "dropped" in the case of a
> miss-identification of a recursion.
>
> Thinking aloud: should we try to move the preemptirq tracers to the C part?


I think we should try to move as much as possible to the C part.