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

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Date:   Tue, 25 Feb 2020 21:43:46 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     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 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()?

(My tracepoint-fu is pretty bad, and I can't actually find where it's
defined as a tracepoint.  But then it does nothing at all, so it must be
a tracepoint, right?)

>  	enter_from_user_mode();
>  	local_irq_enable();
>  }