linux-kernel - Re: [PATCH v2 3/9] rcu,tracing: Create trace_rcu

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Message-ID: <20200213164031.GH14914@hirez.programming.kicks-ass.net>
Date:   Thu, 13 Feb 2020 17:40:31 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        rostedt@...dmis.org, mingo@...nel.org, gregkh@...uxfoundation.org,
        gustavo@...eddedor.com, tglx@...utronix.de, josh@...htriplett.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com
Subject: Re: [PATCH v2 3/9] rcu,tracing: Create trace_rcu_{enter,exit}()

On Thu, Feb 13, 2020 at 05:51:38AM -0800, Paul E. McKenney wrote:

> The reason for the irq argument is to avoid invoking
> rcu_prepare_for_idle() and rcu_dynticks_task_enter() from NMI context
> from rcu_nmi_exit_common().  Similarly, we need to avoid invoking
> rcu_dynticks_task_exit() and rcu_cleanup_after_idle() from NMI context
> from rcu_nmi_enter_common().

Aaah, I see. I didn't grep hard enough earlier today (I only found
stubs). Yes, those take locks, we mustn't call them from NMI context.

> It might well be that I could make these functions be NMI-safe, but
> rcu_prepare_for_idle() in particular would be a bit ugly at best.
> So, before looking into that, I have a question.  Given these proposed
> changes, will rcu_nmi_exit_common() and rcu_nmi_enter_common() be able
> to just use in_nmi()?

That _should_ already be the case today. That is, if we end up in a
tracer and in_nmi() is unreliable we're already screwed anyway.