linux-kernel - Re: [PATCH x86/entry: Force rcu_irq

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Date:   Fri, 12 Jun 2020 12:28:11 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     paulmck@...nel.org, LKML <linux-kernel@...r.kernel.org>,
        rcu@...r.kernel.org, Andrew Lutomirski <luto@...nel.org>,
        X86 ML <x86@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH x86/entry: Force rcu_irq_enter() when in idle task


> On Jun 12, 2020, at 12:25 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> "Paul E. McKenney" <paulmck@...nel.org> writes:
>>> On Fri, Jun 12, 2020 at 10:49:53AM -0700, Paul E. McKenney wrote:
>>> I will therefore address this issue in a follow-on patch.
>> 
>> I should add that -your- patch from yesterday did -not- cause this
>> problem, in case that is of interest.
> 
> So I still can add it back and amend the changelog and comment:
> 
> Change the condition from !rcu_is_watching() to is_idle_task(current) ||
> rcu_is_watching() which enforces that interrupts in the idle task
> unconditionally invoke rcu_irq_enter() independent of the RCU state. For
> most scenarios is_idle_task() would be sufficient but Task RCU needs it
> according to Paul.
> 
> 

Can you easily get a call trace for a case where the system is fully booted, is_idle_task returns false, and rcu_is_watching() also returns false?