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Date:   Tue, 11 Feb 2020 16:31:24 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        "Joel Fernandes (Google)" <joel@joelfernandes.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Gustavo A. R. Silva" <gustavo@embeddedor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Josh Triplett <josh@joshtriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>
Subject: Re: [PATCH] tracing/perf: Move rcu_irq_enter/exit_irqson() to perf
 trace point hook
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On Tue, Feb 11, 2020 at 07:06:15AM -0800, Paul E. McKenney wrote:
> And I have to ask...  What happens if we are very early in from-idle
> NMI entry (or very late in NMI exit), such that both in_nmi() and
> rcu_is_watching() are returning false?  Or did I miss a turn somewhere?

We must, by very careful inspection, ensure that doesn't happen.

No tracing must happen before preempt_count increment / after
preempt_count decrement. Otherwise we can no longer recover.