[PATCH v2] powerpc/smp: do not decrement idle task preempt count in CPU offline

[Srikar Dronamraju]

* Nathan Lynch <nathanl at linux.ibm.com> [2021-10-15 12:39:02]:

> With PREEMPT_COUNT=y, when a CPU is offlined and then onlined again, we
> get:
> 
> BUG: scheduling while atomic: swapper/1/0/0x00000000
> no locks held by swapper/1/0.
> CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.15.0-rc2+ #100
> Call Trace:
>  dump_stack_lvl+0xac/0x108
>  __schedule_bug+0xac/0xe0
>  __schedule+0xcf8/0x10d0
>  schedule_idle+0x3c/0x70
>  do_idle+0x2d8/0x4a0
>  cpu_startup_entry+0x38/0x40
>  start_secondary+0x2ec/0x3a0
>  start_secondary_prolog+0x10/0x14
> 
> This is because powerpc's arch_cpu_idle_dead() decrements the idle task's
> preempt count, for reasons explained in commit a7c2bb8279d2 ("powerpc:
> Re-enable preemption before cpu_die()"), specifically "start_secondary()
> expects a preempt_count() of 0."
> 
> However, since commit 2c669ef6979c ("powerpc/preempt: Don't touch the idle
> task's preempt_count during hotplug") and commit f1a0a376ca0c ("sched/core:
> Initialize the idle task with preemption disabled"), that justification no
> longer holds.
> 
> The idle task isn't supposed to re-enable preemption, so remove the
> vestigial preempt_enable() from the CPU offline path.
> 
> Tested with pseries and powernv in qemu, and pseries on PowerVM.
> 
> Fixes: 2c669ef6979c ("powerpc/preempt: Don't touch the idle task's preempt_count during hotplug")
> Signed-off-by: Nathan Lynch <nathanl at linux.ibm.com>
> Reviewed-by: Valentin Schneider <valentin.schneider at arm.com>

Looks good to me.

Reviewed-by: Srikar Dronamraju <srikar at linux.vnet.ibm.com>

> ---
> 
> Notes:
>     Changes since v1:
>     
>     - remove incorrect Fixes: tag, add Valentin's r-b.
> 
>  arch/powerpc/kernel/smp.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> index 9cc7d3dbf439..605bab448f84 100644
> --- a/arch/powerpc/kernel/smp.c
> +++ b/arch/powerpc/kernel/smp.c
> @@ -1730,8 +1730,6 @@ void __cpu_die(unsigned int cpu)
> 
>  void arch_cpu_idle_dead(void)
>  {
> -	sched_preempt_enable_no_resched();
> -
>  	/*
>  	 * Disable on the down path. This will be re-enabled by
>  	 * start_secondary() via start_secondary_resume() below
> -- 
> 2.31.1
> 

-- 
Thanks and Regards
Srikar Dronamraju