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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Frederic Weisbecker <frederic@kernel.org>
Cc:     "Paul E. McKenney" <paulmck@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, rcu@vger.kernel.org,
        Andrew Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Joel Fernandes <joel@joelfernandes.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH x86/entry: Force rcu_irq_enter() when in idle task
In-Reply-To: <20200612142621.GA8009@lenoir>
References: <20200611235305.GA32342@paulmck-ThinkPad-P72> <CALCETrWo-zpiDsYGtKvm8LzW6CQ5L19a3+Ag_9g8aL4wHaJj9g@mail.gmail.com> <871rmkzcc8.fsf@nanos.tec.linutronix.de> <87wo4cxubv.fsf@nanos.tec.linutronix.de> <20200612142621.GA8009@lenoir>
Date:   Fri, 12 Jun 2020 16:47:44 +0200
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Frederic Weisbecker <frederic@kernel.org> 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" <paulmck@kernel.org>
>> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
>
> Acked-by: Frederic Weisbecker <frederic@kernel.org>
>
> 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().