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Date:   Fri, 12 Jun 2020 08:32:13 -0700
From:   Andy Lutomirski <>
To:     Thomas Gleixner <>
Cc:     "Paul E. McKenney" <>,
        LKML <>,,
        Andrew Lutomirski <>, X86 ML <>,
        Frederic Weisbecker <>,
        Steven Rostedt <>,
        Joel Fernandes <>,
        Mathieu Desnoyers <>,
        Will Deacon <>,
        Peter Zijlstra <>
Subject: Re: [PATCH x86/entry: Force rcu_irq_enter() when in idle task

On Fri, Jun 12, 2020 at 6:55 AM Thomas Gleixner <> wrote:
> The idea of conditionally calling into rcu_irq_enter() only when RCU is
> not watching turned out to be not completely thought through.
> Paul noticed occasional premature end of grace periods in RCU torture
> testing. Bisection led to the commit which made the invocation of
> rcu_irq_enter() conditional on !rcu_is_watching().
> It turned out that this conditional breaks RCU assumptions about the idle
> task when the scheduler tick happens to be a nested interrupt. Nested
> interrupts can happen when the first interrupt invokes softirq processing
> on return which enables interrupts. If that nested tick interrupt does not
> invoke rcu_irq_enter() then the nest accounting in RCU claims that this is
> the first interrupt which might mark a quiescient state and end grace
> periods prematurely.
> Change the condition from !rcu_is_watching() to is_idle_task(current) which
> enforces that interrupts in the idle task unconditionally invoke
> rcu_irq_enter() independent of the RCU state.
> This is also correct vs. user mode entries in NOHZ full scenarios because
> user mode entries bring RCU out of EQS and force the RCU irq nesting state
> accounting to nested. As only the first interrupt can enter from user mode
> a nested tick interrupt will enter from kernel mode and as the nesting
> state accounting is forced to nesting it will not do anything stupid even
> if rcu_irq_enter() has not been invoked.

Acked-by: Andy Lutomirski <>

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