linux-kernel - Re: [patch V4 part 3 11/29] rcu: Provide rcu_irq_exit

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Date:   Wed, 13 May 2020 22:46:51 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        "Paul E. McKenney" <paulmck@...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>,
        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()

On Wed, May 13, 2020 at 10:41:16PM -0400, Joel Fernandes wrote:
> Hi Thomas,
> 
> On Tue, May 05, 2020 at 03:44:05PM +0200, Thomas Gleixner wrote:
> 
> Thank you for CC'ing me.
> 
> > 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.
> 
> Could you let me know which patch or part in the multi-part series is using it?

Ah I see its "x86/entry/common: Provide idtentry_enter/exit()" patch. I'll go
read that tomorrow. Thanks!

 - Joel