Re: [PATCH] s390: preempt: Fix preempt_count initialization

[Heiko Carstens <hca@linux.ibm.com>]

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>
> ---
>  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@linux.ibm.com>

Vasily, can you pick this one up, please?