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

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Date:   Wed, 26 Feb 2020 03:20:25 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        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



> 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:
>>> On 2/25/20 2:08 PM, Thomas Gleixner wrote:
>>> Now that the C entry points are safe, move the irq flags tracing code into
>>> the entry helper.
>> 
>> I'm so confused.
>> 
>>> 
>>> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
>>> ---
>>> arch/x86/entry/common.c          |    5 +++++
>>> arch/x86/entry/entry_32.S        |   12 ------------
>>> arch/x86/entry/entry_64.S        |    2 --
>>> arch/x86/entry/entry_64_compat.S |   18 ------------------
>>> 4 files changed, 5 insertions(+), 32 deletions(-)
>>> 
>>> --- a/arch/x86/entry/common.c
>>> +++ b/arch/x86/entry/common.c
>>> @@ -57,6 +57,11 @@ static inline void enter_from_user_mode(
>>>  */
>>> static __always_inline void syscall_entry_fixups(void)
>>> {
>>> +    /*
>>> +     * Usermode is traced as interrupts enabled, but the syscall entry
>>> +     * mechanisms disable interrupts. Tell the tracer.
>>> +     */
>>> +    trace_hardirqs_off();
>> 
>> 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. And I’m still rather baffled by most of the notrace annotations in the series.