linux-kernel - Re: WARN_ON: userstacktrace on irq events

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Message-ID: <alpine.DEB.2.21.1904051000290.1802@nanos.tec.linutronix.de>
Date:   Fri, 5 Apr 2019 10:12:27 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Steven Rostedt <rostedt@...dmis.org>
cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Clark Williams <williams@...hat.com>,
        Juri Lelli <juri.lelli@...il.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: WARN_ON: userstacktrace on irq events

On Wed, 3 Apr 2019, Steven Rostedt wrote:
> Juri reported this from the -rt kernel, but I can easily trigger it in
> mainline. By simply doing:
> 
>  # cd /sys/kernel/tracing
>  # echo 1 > options/userstacktrace
>  # echo 1 > events/irq/enable
> This is simply caused by the irq trace events doing a user stack trace:
> 
> ftrace_trace_userstack {
>   save_stack_trace_user {
>     __save_stack_trace_user {
>       copy_stack_frame {
>         access_ok {
> 	  WARN_ON_IN_IRQ()
> 
> BOOM! Warn on.
> 
> Can we make that access_ok() call in the copy_stack_frame not trigger
> the warning just if we are in an interrupt?

You really want to have access_ok_atomic() or such which does not have the
WARN and use that in copy_stack_frame(). That's fine here because the
actual copy is inside a pagefault disabled region.

Thanks,

	tglx