Re: [Xen-devel] [PATCH v3] x86: Fix possible ASSERT(cpu < nr_cpu_ids)

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 19.04.18 at 12:20, <Davidwang@xxxxxxxxxxx> wrote:
> From: David Wang <davidwang@xxxxxxxxxxx>
> 
> CPUs may share an in-use channel. Hence clearing of a bit from
> the cpumask (in hpet_broadcast_exit()) as well as setting one
> (in hpet_broadcast_enter()) must not race evaluation of that same
> cpumask. Therefore avoid evaluating the cpumask twice in
> hpet_detach_channel(). Otherwise cpumask_empty() may e.g.return
> false while the subsequent cpumask_first() could return nr_cpu_ids,
> which then triggers the assertion in cpumask_of() reached through
> set_channel_irq_affinity().
> 
> Sign-off-by: David Wang <davidwang@xxxxxxxxxxx>

Signed-off-by

> --- a/xen/arch/x86/hpet.c
> +++ b/xen/arch/x86/hpet.c
> @@ -509,15 +509,18 @@ static void hpet_attach_channel(unsigned int cpu,
>  static void hpet_detach_channel(unsigned int cpu,
>                                  struct hpet_event_channel *ch)
>  {
> +    unsigned int next;
> +
>      spin_lock_irq(&ch->lock);
>  
>      ASSERT(ch == per_cpu(cpu_bc_channel, cpu));
>  
>      per_cpu(cpu_bc_channel, cpu) = NULL;
> +    next = cpumask_first(ch->cpumask);
>  
>      if ( cpu != ch->cpu )
>          spin_unlock_irq(&ch->lock);
> -    else if ( cpumask_empty(ch->cpumask) )
> +    else if ( next == nr_cpu_ids )

This should be >= .

Also I'd prefer if the cpumask_first() was avoided in the cpu != ch->cpu
case.

With these
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
(and the changes are easy enough to make while committing)

Also Cc Jürgen.

Jan


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