Re: [PATCH 04/11] ARM: Allow IPIs to be handled as normal interrupts

[Valentin Schneider]

On 19/05/20 23:24, Russell King - ARM Linux admin wrote:
> On Tue, May 19, 2020 at 05:17:48PM +0100, Marc Zyngier wrote:
>> In order to deal with IPIs as normal interrupts, let's add
>> a new way to register them with the architecture code.
>>
>> set_smp_ipi_range() takes a range of interrupts, and allows
>> the arch code to request them as if the were normal interrupts.
>> A standard handler is then called by the core IRQ code to deal
>> with the IPI.
>>
>> This means that we don't need to call irq_enter/irq_exit, and
>> that we don't need to deal with set_irq_regs either. So let's
>> move the dispatcher into its own function, and leave handle_IPI()
>> as a compatibility function.
>>
>> On the sending side, let's make use of ipi_send_mask, which
>> already exists for this purpose.
>
> You say nothing about the nesting of irq_enter() and irq_exit()
> for scheduler_ipi().
>
> Given that lockdep introduced the requirement that hard IRQs can't
> be nested, are we sure that calling irq_exit() twice is safe?
>
> Looking at irqtime_account_irq(), it seems that will cause double-
> accounting of in-interrupt time, since we will increment
> irq_start_time by just over twice the the period spent handling
> the IPI.
>
> I think the rest of irq_exit() should be safe, but still, this
> behaviour should be documented at the very least, if not avoided.
>

x86 does the same (though IIUC only when tracing reschedule IPI's), and
MIPS has the same issue as it also uses generic IRQ IPI's - so although
it's not ideal, I think we can live with it.