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

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Date:   Mon, 11 Jul 2022 20:53:19 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Paul E. McKenney" <paulmck@...nel.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 Mon, 11 Jul 2022 17:21:28 -0700
"Paul E. McKenney" <paulmck@...nel.org> wrote:

> On x86, both srcu_read_lock() and srcu_read_unlock() should be OK from
> NMI context, give or take their use of lockdep.  Which is why we have
> srcu_read_lock_notrace() and srcu_read_unlock_notrace(), which do not
> use lockdep.  Which __DO_TRACE() does in fact invoke.  Ah, but you have
> this: "WARN_ON_ONCE(rcuidle && in_nmi())".
> 
> Because all the world is not an x86.

But since NMIs are architecture specific, we could change that to:

	WARN_ON_ONCE(!srcu_nmi_safe && rcuidle && in_nmi());

and add a srcu_nmi_safe constant or macro that is 1 on architectures that
srcu is safe in NMI and 0 otherwise.

Or do we care if a tracepoint happens in those architectures where it is
not safe. We could then just do:

	if (!srcu_nmi_safe && rcuidle && in_nmi())
		return;

and just skip tracepoints that are marked rcu_idle and happen within NMI.

-- Steve