linux-kernel - Re: [patch 4/8] x86/entry: Move irq tracing on syscall entry to C-code

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Message-ID: <87sgixb00q.fsf@nanos.tec.linutronix.de>
Date:   Wed, 26 Feb 2020 20:51:01 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...capital.net>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Steven Rostedt <rostedt@...dmis.org>,
        Brian Gerst <brgerst@...il.com>,
        Juergen Gross <JGross@...e.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [patch 4/8] x86/entry: Move irq tracing on syscall entry to C-code

Andy Lutomirski <luto@...capital.net> writes:
>> On Feb 26, 2020, at 12:17 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>> On Tue, Feb 25, 2020 at 09:43:46PM -0800, Andy Lutomirski wrote:
>>> Your earlier patches suggest quite strongly that tracing isn't safe
>>> until enter_from_user_mode().  But trace_hardirqs_off() calls
>>> trace_irq_disable_rcuidle(), which looks [0] like a tracepoint.
>>> 
>>> Did you perhaps mean to do this *after* enter_from_user_mode()?
>> 
>> aside from the fact that enter_from_user_mode() itself also has a
>> tracepoint, the crucial detail is that we must not trace/kprobe the
>> function calling this.
>> 
>> Specifically for #PF, because we need read_cr2() before this. See later
>> patches.
>
> Indeed. I’m fine with this patch, but I still don’t understand what
> the changelog is about.

Yeah, the changelog is not really helpful. Let me fix that.

> And I’m still rather baffled by most of the notrace annotations in the
> series.

As discussed on IRC, this might be too broad, but then I rather have the
actual C-entry points neither traceable nor probable in general and
relax this by calling functions which can be traced and probed.

My rationale for this decision was that enter_from_user_mode() is marked
notrace/noprobe as well, so I kept the protection scope the same as we
had in the ASM maze which is marked noprobe already.

Thanks,

        tglx