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SubjectRe: [PATCH v17 2/8] cpu/hotplug: Reset task stack state in _cpu_up()
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On 28/03/2023 20:57, Usama Arif wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
>
> Commit dce1ca0525bf ("sched/scs: Reset task stack state in bringup_cpu()")
> ensured that the shadow call stack was reset and KASAN poisoning removed
> from a CPU's stack each time that CPU is brought up, not just once.
>
> This is not incorrect. However, with parallel bringup, an architecture
> may obtain the idle thread for a new CPU from a pre-bringup stage, by
> calling idle_thread_get() for itself. This would mean that the cleanup
> in bringup_cpu() would be too late.
>
> Move the SCS/KASAN cleanup to the generic _cpu_up() function instead,
> which already ensures that the new CPU's stack is available, purely to
> allow for early failure. This occurs when the CPU to be brought up is
> in the CPUHP_OFFLINE state, which should correctly do the cleanup any
> time the CPU has been taken down to the point where such is needed.
>
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> Reviewed-by: Mark Rutland <mark.rutland@arm.com>
> Tested-by: Mark Rutland <mark.rutland@arm.com> [arm64]

Forgot to include my sign-off. Thanks David for pointing it out.

Signed-off-by: Usama Arif <usama.arif@bytedance.com>

> ---
> kernel/cpu.c | 12 ++++++------
> 1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 6c0a92ca6bb5..43e0a77f21e8 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -591,12 +591,6 @@ static int bringup_cpu(unsigned int cpu)
> struct task_struct *idle = idle_thread_get(cpu);
> int ret;
>
> - /*
> - * Reset stale stack state from the last time this CPU was online.
> - */
> - scs_task_reset(idle);
> - kasan_unpoison_task_stack(idle);
> -
> /*
> * Some architectures have to walk the irq descriptors to
> * setup the vector space for the cpu which comes online.
> @@ -1383,6 +1377,12 @@ static int _cpu_up(unsigned int cpu, int tasks_frozen, enum cpuhp_state target)
> ret = PTR_ERR(idle);
> goto out;
> }
> +
> + /*
> + * Reset stale stack state from the last time this CPU was online.
> + */
> + scs_task_reset(idle);
> + kasan_unpoison_task_stack(idle);
> }
>
> cpuhp_tasks_frozen = tasks_frozen;

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