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Date:   Fri, 12 Jun 2020 16:47:44 +0200
From:   Thomas Gleixner <>
To:     Frederic Weisbecker <>
Cc:     "Paul E. McKenney" <>,
        LKML <>,,
        Andrew Lutomirski <>, X86 ML <>,
        Steven Rostedt <>,
        Joel Fernandes <>,
        Mathieu Desnoyers <>,
        Will Deacon <>,
        Peter Zijlstra <>
Subject: Re: [PATCH x86/entry: Force rcu_irq_enter() when in idle task

Frederic Weisbecker <> writes:

> On Fri, Jun 12, 2020 at 03:55:00PM +0200, 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.
>> Fixes: 3eeec3858488 ("x86/entry: Provide idtentry_entry/exit_cond_rcu()")
>> Reported-by: "Paul E. McKenney" <>
>> Signed-off-by: Thomas Gleixner <>
> Acked-by: Frederic Weisbecker <>
> So, in the end the call to rcu_irq_enter() in irq_enter() is going to
> be useless in x86, right?

x86 is not calling irq_enter() anymore. It's using irq_enter_rcu().

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