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Date:   Thu, 15 Dec 2022 15:10:27 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Masami Hiramatsu <mhiramat@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux Trace Kernel <linux-trace-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Subject: Re: [PATCH] tracing: Do not synchronize freeing of trigger filter on
 boot up
Message-ID: <20221215231027.GS4001@paulmck-ThinkPad-P17-Gen-1>
Reply-To: paulmck@kernel.org
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On Thu, Dec 15, 2022 at 05:39:13PM -0500, Steven Rostedt wrote:
> On Thu, 15 Dec 2022 11:01:58 -0800
> "Paul E. McKenney" <paulmck@kernel.org> wrote:
> 
> > What case?
> > 
> > Here is one:
> > 
> > o	The newly spawned init process does something that uses RCU,
> > 	but is preempted while holding rcu_read_lock().
> > 
> > o	The boot thread, which did the preempting, waits for a grace
> > 	period.  If we use rcu_scheduler_active, all is well because
> > 	synchronize_rcu() will do a real run-time grace period, thus
> > 	waiting for that reader.
> > 
> > 	But system_state has not yet been updated, so if synchronize_rcu()
> > 	were instead to pay attention to that one, there might be a
> > 	tragically too-short RCU grace period.
> 
> The thing is, preemption is disabled the entire time here.
> 
> That is, from:
> 
> void kthread_show_list(void);
> noinline void __ref rest_init(void)
> {
> 	struct task_struct *tsk;
> 	int pid;
> 
> 	rcu_scheduler_starting();
> 
> through:
> 
> 	system_state = SYSTEM_SCHEDULING;
> 
> 	complete(&kthreadd_done);
> 
> 
> Preemption is disabled and other CPUs have not even been started yet.
> 
> Although the might_sleep() call might schedule the kernel_init() task but
> that will only block on the completion.
> 
> In other words, I don't think anything can cause any issues this early in
> the boot up.

The nice thing about the current placement of rcu_scheduler_starting()
is that there is not yet any other task to switch to.  ;-)

							Thanx, Paul