linux-kernel - Re: [PATCH linux-next] powerpc: use raw_smp_processor_id in arch_touch_nmi

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Message-ID: <87pmi77w82.fsf@jogness.linutronix.de>
Date:   Thu, 14 Jul 2022 13:52:37 +0206
From:   John Ogness <john.ogness@...utronix.de>
To:     Zhouyi Zhou <zhouzhouyi@...il.com>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        paulus@...ba.org, Nicholas Piggin <npiggin@...il.com>,
        ldufour@...ux.ibm.com, pmladek@...e.com, Julia.Lawall@...ia.fr,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-kernel <linux-kernel@...r.kernel.org>, lance@...osl.org,
        "Paul E. McKenney" <paulmck@...nel.org>, rcu <rcu@...r.kernel.org>
Subject: Re: [PATCH linux-next] powerpc: use raw_smp_processor_id in
 arch_touch_nmi_watchdog

On 2022-07-14, Zhouyi Zhou <zhouzhouyi@...il.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