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Date:   Wed, 06 May 2020 00:05:04 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     paulmck@...nel.org
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Brian Gerst <brgerst@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [patch V4 part 3 11/29] rcu: Provide rcu_irq_exit_preempt()

"Paul E. McKenney" <paulmck@...nel.org> writes:

> On Tue, May 05, 2020 at 03:44:05PM +0200, Thomas Gleixner wrote:
>> Interrupts and exceptions invoke rcu_irq_enter() on entry and need to
>> invoke rcu_irq_exit() before they either return to the interrupted code or
>> invoke the scheduler due to preemption.
>> 
>> The general assumption is that RCU idle code has to have preemption
>> disabled so that a return from interrupt cannot schedule. So the return
>> from interrupt code invokes rcu_irq_exit() and preempt_schedule_irq().
>> 
>> If there is any imbalance in the rcu_irq/nmi* invocations or RCU idle code
>> had preemption enabled then this goes unnoticed until the CPU goes idle or
>> some other RCU check is executed.
>> 
>> Provide rcu_irq_exit_preempt() which can be invoked from the
>> interrupt/exception return code in case that preemption is enabled. It
>> invokes rcu_irq_exit() and contains a few sanity checks in case that
>> CONFIG_PROVE_RCU is enabled to catch such issues directly.
>> 
>> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
>> Cc: "Paul E. McKenney" <paulmck@...nel.org>
>> Cc: Joel Fernandes <joel@...lfernandes.org>
>
> The ->dynticks_nmi_nesting field is going away at some point, but
> there is always "git merge".  ;-)

Yes. The logistics for merging all of this is going to be interesting :)

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