[PATCH] arm: smp: Avoid false positive CPU hotplug Lockdep-RCU splat

[Russell King (Oracle)]

On Sat, Mar 09, 2024 at 08:45:35AM +0100, Stefan Wiehler wrote:
> > I agree with the problem but disagree with the patch because it feels like a
> > terrible workaround.
> > 
> > Can we just use arch_spin_lock() for the cpu_asid_lock? This might require
> > acquiring the raw_lock within the raw_spinlock_t, but there is precedent:
> > 
> > arch/powerpc/kvm/book3s_hv_rm_mmu.c:245:
> > arch_spin_lock(&kvm->mmu_lock.rlock.raw_lock);
> > 
> > IMO, lockdep tracking of this lock is not necessary or possible considering the
> > hotplug situation.
> > 
> > Or is there a reason you need lockdep working for the cpu_asid_lock?
> 
> I was not aware of this possibility to bypass lockdep tracing, but this seems
> to work and indeed looks like less of a workaround:
> 
> diff --git a/arch/arm/mm/context.c b/arch/arm/mm/context.c
> index 4204ffa2d104..4fc2c559f1b6 100644
> --- a/arch/arm/mm/context.c
> +++ b/arch/arm/mm/context.c
> @@ -254,7 +254,8 @@ void check_and_switch_context(struct mm_struct *mm, struct task_struct *tsk)
>             && atomic64_xchg(&per_cpu(active_asids, cpu), asid))
>                 goto switch_mm_fastpath;
> 
> -       raw_spin_lock_irqsave(&cpu_asid_lock, flags);
> +       local_irq_save(flags);
> +       arch_spin_lock(&cpu_asid_lock.raw_lock);
>         /* Check that our ASID belongs to the current generation. */
>         asid = atomic64_read(&mm->context.id);
>         if ((asid ^ atomic64_read(&asid_generation)) >> ASID_BITS) {
> @@ -269,7 +270,8 @@ void check_and_switch_context(struct mm_struct *mm, struct task_struct *tsk)
> 
>         atomic64_set(&per_cpu(active_asids, cpu), asid);
>         cpumask_set_cpu(cpu, mm_cpumask(mm));
> -       raw_spin_unlock_irqrestore(&cpu_asid_lock, flags);
> +       arch_spin_unlock(&cpu_asid_lock.raw_lock);
> +       local_irq_restore(flags);
> 
>  switch_mm_fastpath:
>         cpu_switch_mm(mm->pgd, mm);
> 
> @Russell, what do you think?

I think this is Will Deacon's code, so we ought to hear from Will...

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