Re: [Xen-devel] [PATCH v3 4/5] x86/smp: use a dedicated scratch cpumask in send_IPI_mask

[Jan Beulich <jbeulich@suse.com>]

On 26.02.2020 11:18, Roger Pau Monné wrote:
> On Wed, Feb 26, 2020 at 11:07:44AM +0100, Jan Beulich wrote:
>> On 24.02.2020 11:46, Roger Pau Monne wrote:
>>> Using scratch_cpumask in send_IPI_mask is not safe because it can be
>>> called from interrupt context, and hence Xen would have to make sure
>>> all the users of the scratch cpumask disable interrupts while using
>>> it.
>>>
>>> Instead introduce a new cpumask to be used by send_IPI_mask, and
>>> disable interrupts while using it.
>>
>> The alternative of also adding ...
>>
>>> --- a/xen/arch/x86/smp.c
>>> +++ b/xen/arch/x86/smp.c
>>> @@ -59,6 +59,7 @@ static void send_IPI_shortcut(unsigned int shortcut, int vector,
>>>      apic_write(APIC_ICR, cfg);
>>>  }
>>>  
>>> +DECLARE_PER_CPU(cpumask_var_t, send_ipi_cpumask);
>>>  /*
>>>   * send_IPI_mask(cpumask, vector): sends @vector IPI to CPUs in @cpumask,
>>>   * excluding the local CPU. @cpumask may be empty.
>>> @@ -67,7 +68,21 @@ static void send_IPI_shortcut(unsigned int shortcut, int vector,
>>>  void send_IPI_mask(const cpumask_t *mask, int vector)
>>>  {
>>>      bool cpus_locked = false;
>>> -    cpumask_t *scratch = this_cpu(scratch_cpumask);
>>> +    cpumask_t *scratch = this_cpu(send_ipi_cpumask);
>>> +    unsigned long flags;
>>> +
>>> +    if ( in_mc() || in_nmi() )
>>
>> ... in_irq() here was considered, and discarded because of? With
>> this you'd not need the second CPU mask and you'd also be able
>> to avoid disabling an re-enabling IRQs.
> 
> I aimed to use the shorthand as much as possible, but I would also be
> fine with not using it in irq context. I assume there aren't that many
> flushes in irq context, and then the IPIs sent are probably not
> broadcast ones.

Well, here it's not just flushes, is it? Knowing some (typical)
IRQ context uses could allow us take a better decision. After
Sander's report, did you actually identify what path(s) the
earlier change broke?

Jan

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