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Sender: Guenter Roeck <groeck7@gmail.com>
Date:   Wed, 7 Jul 2021 12:42:26 -0700
From:   Guenter Roeck <linux@roeck-us.net>
To:     Valentin Schneider <valentin.schneider@arm.com>
Cc:     linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Alexey Kardashevskiy <aik@ozlabs.ru>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH] s390: preempt: Fix preempt_count initialization
Message-ID: <20210707194226.GA2318012@roeck-us.net>
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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@roeck-us.net>
> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>

Tested-by: Guenter Roeck <linux@roeck-us.net>

Guenter