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From:   John Ogness <john.ogness@linutronix.de>
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To:     Zhouyi Zhou <zhouzhouyi@gmail.com>
Cc:     Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        paulus@samba.org, Nicholas Piggin <npiggin@gmail.com>,
        ldufour@linux.ibm.com, pmladek@suse.com, Julia.Lawall@inria.fr,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        linux-kernel <linux-kernel@vger.kernel.org>, lance@osuosl.org,
        "Paul E. McKenney" <paulmck@kernel.org>, rcu <rcu@vger.kernel.org>
Subject: Re: [PATCH linux-next] powerpc: use raw_smp_processor_id in
 arch_touch_nmi_watchdog
In-Reply-To: <CAABZP2x-OiwX7-nRvdPz0NU1D5kgUoL4MQOgTqLK1KgLcrZ5DA@mail.gmail.com>
References: <20220714013131.12648-1-zhouzhouyi@gmail.com>
 <87tu7k6o7n.fsf@jogness.linutronix.de>
 <CAABZP2x-OiwX7-nRvdPz0NU1D5kgUoL4MQOgTqLK1KgLcrZ5DA@mail.gmail.com>
Date:   Thu, 14 Jul 2022 13:52:37 +0206
Message-ID: <87pmi77w82.fsf@jogness.linutronix.de>
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On 2022-07-14, Zhouyi Zhou <zhouzhouyi@gmail.com> wrote:
> Thank John for correcting me ;-)

After looking more closely, I do not think disabling migration is the
correct fix either.

The per-cpu variable @wd_timer_tb is written from 2 functions:

- watchdog_timer_interrupt() <-- irq handler
- arch_touch_nmi_watchdog()  <-- called from preemptible

Since watchdog_timer_interrupt() is called from irq context, I expect
that interrupts need to be disabled for the update in
arch_touch_nmi_watchdog(). Perhaps a using a per-cpu local_lock_t with
local_lock_irqsave() to protect write access to @wd_timer_tb?

John Ogness