linux-kernel - Re: [for-next][PATCH 13/25] x86/mm/kmmio: Use rcu_read_lock_sched

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Message-ID: <20221210173227.7063269d@gandalf.local.home>
Date:   Sat, 10 Dec 2022 17:32:27 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Karol Herbst <karolherbst@...il.com>,
        Pekka Paalanen <ppaalanen@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>
Subject: Re: [for-next][PATCH 13/25] x86/mm/kmmio: Use
 rcu_read_lock_sched_notrace()

On Sat, 10 Dec 2022 13:34:12 -0800
"Paul E. McKenney" <paulmck@...nel.org> wrote:

> > I was going to remove it, but then I realized that it would be a functional
> > change, as from the comment above, it uses "preempt_enable_no_resched(),
> > which there is not a rcu_read_unlock_sched() variant.  
> 
> If this happens often enough, it might be worth adding something like
> rcu_read_unlock_sched_no_resched(), but we clearly are not there yet.
> Especially not with a name like that!  ;-)

Please don't ;-)

This is only to handle the bizarre case that mmio tracing does. Remember,
this tracer is only for those that want to reverse engineer a binary
driver. It's not even SMP safe! When you enable it, it shuts down all but
one CPU. This is actually the reason I worked so hard to keep it working
with lockdep. The shutting down of CPUs has caught so many bugs in other
parts of the kernel! ;-)

Thus, anything that mmio tracer does, is considered niche, and not
something to much care about.

-- Steve