linux-kernel - Re: [PATCH] s390: preempt: Fix preempt

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Date:   Thu, 8 Jul 2021 16:17:10 +0200
From:   Heiko Carstens <hca@...ux.ibm.com>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
        Guenter Roeck <linux@...ck-us.net>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Alexey Kardashevskiy <aik@...abs.ru>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] s390: preempt: Fix preempt_count initialization

On Wed, Jul 07, 2021 at 05:33:38PM +0100, Valentin Schneider wrote:
> S390's init_idle_preempt_count(p, cpu) doesn't actually let us initialize the
> preempt_count of the requested CPU's idle task: it unconditionally writes
> to the current CPU's. This clearly conflicts with idle_threads_init(),
> which intends to initialize *all* the idle tasks, including their
> preempt_count (or their CPU's, if the arch uses a per-CPU preempt_count).
> 
> Unfortunately, it seems the way s390 does things doesn't let us initialize
> every possible CPU's preempt_count early on, as the pages where this
> resides are only allocated when a CPU is brought up and are freed when it
> is brought down.
> 
> Let the arch-specific code set a CPU's preempt_count when its lowcore is
> allocated, and turn init_idle_preempt_count() into an empty stub.
> 
> Fixes: f1a0a376ca0c ("sched/core: Initialize the idle task with preemption disabled")
> Reported-by: Guenter Roeck <linux@...ck-us.net>
> Signed-off-by: Valentin Schneider <valentin.schneider@....com>
> ---
>  arch/s390/include/asm/preempt.h | 16 ++++------------
>  arch/s390/kernel/setup.c        |  1 +
>  arch/s390/kernel/smp.c          |  1 +
>  3 files changed, 6 insertions(+), 12 deletions(-)

Reviewed-by: Heiko Carstens <hca@...ux.ibm.com>

Vasily, can you pick this one up, please?