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

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Message-ID: <20221211055218.GH4001@paulmck-ThinkPad-P17-Gen-1>
Date:   Sat, 10 Dec 2022 21:52:18 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.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, Dec 10, 2022 at 05:32:27PM -0500, Steven Rostedt wrote:
> 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.

Agreed, as I said, we are clearly not there yet.

							Thanx, Paul