Subject: Re: [PATCH] arm64: abort counter_read_on_cpu() when irqs_disabled()

From: Ionela Voinescu <[email-masked]>
Date: Fri, 13 Nov 2020 16:58:43 +0000

On Friday 13 Nov 2020 at 16:02:34 (+0000), Mark Rutland wrote:
> On Fri, Nov 13, 2020 at 03:53:28PM +0000, Ionela Voinescu wrote:
> > Given that smp_call_function_single() can deadlock when interrupts are
> > disabled, abort the SMP call if irqs_disabled(). This scenario is
> > currently not possible given the function's uses, but safeguard this for
> > potential future uses.
> 
> Sorry to contradict earlier feedback, but I think this is preferable
> as-is, since smp_call_function_single() will
> WARN_ON_ONCE(irqs_disabled())), but this will silently mask any dodgy
> usage.

Probably it only contradicts the chosen implementation.

> 
> If we want a separate check here, I reckon we should wrap it with a
> WARN_ON_ONCE(), and only relax that if/when we have a legitimate case
> for calling this with IRQs disabled.
> 

That's fair. I'll replace the condition below with:

	if (!cpu_has_amu_feat(cpu))
		return -EOPNOTSUPP;

	if (WARN_ON_ONCE(irqs_disabled())
		return -EPERM;

Thanks for your time,
Ionela.

> Thanks,
> Mark.
> 
> > 
> > Signed-off-by: Ionela Voinescu <[email-masked]>
> > Cc: Catalin Marinas <[email-masked]>
> > Cc: Will Deacon <[email-masked]>
> > ---
> >  arch/arm64/kernel/topology.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
> > index 3a083a9a8ef2..e387188741f2 100644
> > --- a/arch/arm64/kernel/topology.c
> > +++ b/arch/arm64/kernel/topology.c
> > @@ -343,7 +343,11 @@ static void cpu_read_constcnt(void *val)
> >  static inline
> >  int counters_read_on_cpu(int cpu, smp_call_func_t func, u64 *val)
> >  {
> > -	if (!cpu_has_amu_feat(cpu))
> > +	/*
> > +	 * Abort call on counterless CPU or when interrupts are
> > +	 * disabled - can lead to deadlock in smp sync call.
> > +	 */
> > +	if (!cpu_has_amu_feat(cpu) || unlikely(irqs_disabled()))
> >  		return -EOPNOTSUPP;
> >  
> >  	smp_call_function_single(cpu, func, val, 1);
> > -- 
> > 2.17.1
> > 

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