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

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Message-ID: <20220712134916.GT1790663@paulmck-ThinkPad-P17-Gen-1>
Date:   Tue, 12 Jul 2022 06:49:16 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Marco Elver <elver@...gle.com>,
        John Ogness <john.ogness@...utronix.de>,
        Petr Mladek <pmladek@...e.com>,
        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>,
        Linux Kernel Functional Testing <lkft@...aro.org>
Subject: Re: [PATCH -printk] printk, tracing: fix console tracepoint

On Tue, Jul 12, 2022 at 09:39:40AM -0400, Steven Rostedt wrote:
> On Tue, 12 Jul 2022 04:49:54 -0700
> "Paul E. McKenney" <paulmck@...nel.org> wrote:
> 
> > > But a quick fix that stopped the bleeding and allowed printk() to
> > > progress would be useful in the short term, regardless of whether or
> > > not in the longer term it makes sense to make srcu_read_lock_trace()
> > > and srcu_read_unlock_trace() NMI-safe.  
> > 
> > Except that doesn't rcuidle && in_nmi() imply a misplaced trace event?
> > 
> > Isn't it still the case that you are not supposed to have trace events
> > in NMI handlers before RCU is watching or after it is no longer watching,
> > just as for entry/exit code in general?  Once in the body of the handler,
> > rcuidle should be false and all should be well.
> > 
> > Or am I missing something here?
> 
> 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()?

							Thanx, Paul