linux-kernel - Re: [PATCH -printk] printk, tracing: fix console tracepoint

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Date:   Tue, 12 Jul 2022 17:25:37 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Marco Elver <elver@...gle.com>,
        John Ogness <john.ogness@...utronix.de>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Johannes Berg <johannes.berg@...el.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Naresh Kamboju <naresh.kamboju@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux Kernel Functional Testing <lkft@...aro.org>
Subject: Re: [PATCH -printk] printk, tracing: fix console tracepoint

On Tue 2022-07-12 10:53:53, Steven Rostedt wrote:
> On Tue, 12 Jul 2022 06:49:16 -0700
> "Paul E. McKenney" <paulmck@...nel.org> wrote:
> 
> > > I guess the question is, can we have printk() in such a place? Because this
> > > tracepoint is attached to printk and where ever printk is done so is this
> > > tracepoint.  
> > 
> > As I understand it, code in such a place should be labeled noinstr.
> > Then the call to printk() would be complained about as an illegal
> > noinstr-to-non-noinstr call.
> > 
> > But where exactly is that printk()?
> 
> Perhaps the fix is to remove the _rcuidle() from trace_console_rcuidle().
> If printk() can never be called from noinstr (aka RCU not watching).

Good point!

My understanding is that printk() should not get called when rcu
is not watching. But it might need to reduce the scope of the code
when rcu is not watching.

PeterZ actually removed _rcuidle() in this trace a patchset,
see https://lore.kernel.org/r/20220608144517.444659212@infradead.org.

Adding Peter into Cc.

Best Regards,
Petr