RE: [Linuxarm] Re: [PATCH for-next 00/32] spin lock usage optimization for SCSI drivers

Finn Thain
2021-02-10T22:37:25Z

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On Wed, 10 Feb 2021, Song Bao Hua (Barry Song) wrote:

> > On Wed, 10 Feb 2021, Song Bao Hua (Barry Song) wrote:
> >
> > > >
> > > > There is no warning from m68k builds. That's because
> > > > arch_irqs_disabled() returns true when the IPL is non-zero.
> > >
> > > So for m68k, the case is
> > > arch_irqs_disabled() is true, but interrupts can still come?
> > >
> > > Then it seems it is very confusing. If prioritized interrupts can
> > > still come while arch_irqs_disabled() is true,
> >
> > Yes, on m68k CPUs, an IRQ having a priority level higher than the
> > present priority mask will get serviced.
> >
> > Non-Maskable Interrupt (NMI) is not subject to this rule and gets
> > serviced regardless.
> >
> > > how could spin_lock_irqsave() block the prioritized interrupts?
> >
> > It raises the the mask level to 7. Again, please see
> > arch/m68k/include/asm/irqflags.h
>
> Hi Finn,
> Thanks for your explanation again.
>
> TBH, that is why m68k is so confusing. irqs_disabled() on m68k should
> just reflect the status of all interrupts have been disabled except NMI.
>
> irqs_disabled() should be consistent with the calling of APIs such as
> local_irq_disable, local_irq_save, spin_lock_irqsave etc.
>

When irqs_disabled() returns true, we cannot infer that
arch_local_irq_disable() was called. But I have not yet found driver code
or core kernel code attempting that inference.

> >
> > > Isn't arch_irqs_disabled() a status reflection of irq disable API?
> > >
> >
> > Why not?
>
> If so, arch_irqs_disabled() should mean all interrupts have been masked
> except NMI as NMI is unmaskable.
>

Can you support that claim with a reference to core kernel code or
documentation? (If some arch code agrees with you, that's neither here nor
there.)

> >
> > Are all interrupts (including NMI) masked whenever
> > arch_irqs_disabled() returns true on your platforms?
>
> On my platform, once irqs_disabled() is true, all interrupts are masked
> except NMI. NMI just ignore spin_lock_irqsave or local_irq_disable.
>
> On ARM64, we also have high-priority interrupts, but they are running as
> PESUDO_NMI:
> https://lwn.net/Articles/755906/
>

A glance at the ARM GIC specification suggests that your hardware works
much like 68000 hardware.

When enabled, a CPU interface takes the highest priority pending
interrupt for its connected processor and determines whether the
interrupt has sufficient priority for it to signal the interrupt
request to the processor. [...]

When the processor acknowledges the interrupt at the CPU interface, the
Distributor changes the status of the interrupt from pending to either
active, or active and pending. At this point the CPU interface can
signal another interrupt to the processor, to preempt interrupts that
are active on the processor. If there is no pending interrupt with
sufficient priority for signaling to the processor, the interface
deasserts the interrupt request signal to the processor.

https://developer.arm.com/documentation/ihi0048/b/

Have you considered that Linux/arm might benefit if it could fully exploit
hardware features already available, such as the interrupt priority
masking feature in the GIC in existing arm systems?

> On m68k, it seems you mean：
> irq_disabled() is true, but high-priority interrupts can still come;
> local_irq_disable() can disable high-priority interrupts, and at that
> time, irq_disabled() is also true.
>
> TBH, this is wrong and confusing on m68k.
>

Like you, I was surprised when I learned about it. But that doesn't mean
it's wrong. The fact that it works should tell you something.

Things could always be made simpler. But discarding features isn't
necessarily an improvement.

> >
> > > Thanks
> > > Barry
> > >
>
> Thanks
> Barry
>