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

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Date:   Wed, 13 Jul 2022 07:05:50 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Steven Rostedt <rostedt@...dmis.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>,
        Linux Kernel Functional Testing <lkft@...aro.org>
Subject: Re: [PATCH -printk] printk, tracing: fix console tracepoint

On Wed, Jul 13, 2022 at 01:25:41PM +0200, Petr Mladek wrote:
> On Tue 2022-07-12 08:16:55, Paul E. McKenney wrote:
> > On Tue, Jul 12, 2022 at 10:53:53AM -0400, 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).
> > 
> > Maybe printk() is supposed to be invoked from noinstr.  It might be a
> > special case in the tooling.  I have no idea.  ;-)
> 
> I think that it is ok to do _not_ support printk() in noinstr parts.
> 
> > However, the current SRCU read-side algorithm will tolerate being invoked
> > from noinstr as long as it is not also an NMI handler.  Much though
> > debugging tools might (or might not) complain.
> > 
> > Don't get me wrong, I can make SRCU tolerate being called while RCU is
> > not watching.  It is not even all that complicated.  The cost is that
> > architectures that have NMIs but do not have NMI-safe this_cpu*()
> > operations have an SRCU reader switch from explicit smp_mb() and
> > interrupt disabling to a cmpxchg() loop relying on the implicit barriers
> > in cmpxchg().
> > 
> > For arm64, this was reportedly a win.
> 
> IMHO, the tracepoint in printk() is not worth slowing down other
> important fast paths.
> 
> The tracepoint was moved into vprintk_store() in 5.19-rc1. It used
> to be in console_unlock() before. The previous location was not
> reliable by definition. Old messages might be overridden by new
> ones before they reach console. Also messages in NMI context
> used to be stored in per-CPU buffers. There was even bigger
> risk that they would not reach the console.

Fair enough, works for me!

							Thanx, Paul