Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Sat, 10 Dec 2022 17:32:27 -0500
From:   Steven Rostedt <rostedt@goodmis.org>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     linux-kernel@vger.kernel.org,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Karol Herbst <karolherbst@gmail.com>,
        Pekka Paalanen <ppaalanen@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@alien8.de>
Subject: Re: [for-next][PATCH 13/25] x86/mm/kmmio: Use
 rcu_read_lock_sched_notrace()
Message-ID: <20221210173227.7063269d@gandalf.local.home>
In-Reply-To: <20221210213412.GF4001@paulmck-ThinkPad-P17-Gen-1>
References: <20221210135750.425719934@goodmis.org>
        <20221210135825.241167123@goodmis.org>
        <20221210174753.GD4001@paulmck-ThinkPad-P17-Gen-1>
        <20221210133425.4657985e@gandalf.local.home>
        <20221210213412.GF4001@paulmck-ThinkPad-P17-Gen-1>
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Precedence: bulk

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