Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable
From:   Andy Lutomirski <luto@amacapital.net>
Mime-Version: 1.0 (1.0)
Subject: Re: [PATCH x86/entry: Force rcu_irq_enter() when in idle task
Date:   Fri, 12 Jun 2020 12:28:11 -0700
Message-Id: <FE145E46-D160-4826-9D81-553EB2A660C1@amacapital.net>
References: <87r1ukxf1b.fsf@nanos.tec.linutronix.de>
Cc:     paulmck@kernel.org, LKML <linux-kernel@vger.kernel.org>,
        rcu@vger.kernel.org, Andrew Lutomirski <luto@kernel.org>,
        X86 ML <x86@kernel.org>,
        Frederic Weisbecker <frederic@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>
In-Reply-To: <87r1ukxf1b.fsf@nanos.tec.linutronix.de>
To:     Thomas Gleixner <tglx@linutronix.de>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Jun 12, 2020, at 12:25 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
>=20
> =EF=BB=BF"Paul E. McKenney" <paulmck@kernel.org> writes:
>>> On Fri, Jun 12, 2020 at 10:49:53AM -0700, Paul E. McKenney wrote:
>>> I will therefore address this issue in a follow-on patch.
>>=20
>> I should add that -your- patch from yesterday did -not- cause this
>> problem, in case that is of interest.
>=20
> So I still can add it back and amend the changelog and comment:
>=20
> Change the condition from !rcu_is_watching() to is_idle_task(current) ||
> rcu_is_watching() which enforces that interrupts in the idle task
> unconditionally invoke rcu_irq_enter() independent of the RCU state. For
> most scenarios is_idle_task() would be sufficient but Task RCU needs it
> according to Paul.
>=20
>=20

Can you easily get a call trace for a case where the system is fully booted,=
 is_idle_task returns false, and rcu_is_watching() also returns false?=