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

[Russell King (Oracle)]

On Tue, Mar 12, 2024 at 10:14:40PM +0000, Will Deacon wrote:
> Hi Russell,
> 
> On Sat, Mar 09, 2024 at 09:57:04AM +0000, Russell King (Oracle) wrote:
> > On Sat, Mar 09, 2024 at 08:45:35AM +0100, Stefan Wiehler wrote:
> > > 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...
> 
> Thanks for adding me in.
> 
> Using arch_spin_lock() really feels like a bodge to me. This code isn't
> run only on the hot-unplug path, but rather this is part of switch_mm()
> and we really should be able to have lockdep work properly there for
> the usual case.
> 
> Now, do we actually need to worry about the ASID when switching to the
> init_mm? I'd have thought that would be confined to global (kernel)
> mappings, so I wonder whether we could avoid this slow path code
> altogether like we do on arm64 in __switch_mm(). But I must confess that
> I don't recall the details of the pre-LPAE MMU configuration...

As the init_mm shouldn't have any userspace mappings, isn't the ASID
entirely redundant? Couldn't check_and_switch_context() just simply
do the vmalloc seq check, set the reserved ASID, and then head to
switch_mm_fastpath to call the mm switch code?

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