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

Finn Thain
2021-02-10T21:07:54Z

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

> > On Tue, 9 Feb 2021, Song Bao Hua (Barry Song) wrote:
> >
> > > > > sonic_interrupt() uses an irq lock within an interrupt handler
> > > > > to avoid issues relating to this. This kind of locking may be
> > > > > needed in the drivers you are trying to patch. Or it might not.
> > > > > Apparently, no-one has looked.
> > >
> > > Is the comment in sonic_interrupt() outdated according to this:
> > > m68k: irq: Remove IRQF_DISABLED
> > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=77a4279
> > > http://lkml.iu.edu/hypermail/linux/kernel/1109.2/01687.html
> > >
> >
> > The removal of IRQF_DISABLED isn't relevant to this driver. Commit
> > 77a42796786c ("m68k: Remove deprecated IRQF_DISABLED") did not disable
> > interrupts, it just removed some code to enable them.
> >
> > The code and comments in sonic_interrupt() are correct. You can
> > confirm this for yourself quite easily using QEMU and a
> > cross-compiler.
> >
> > > and this: genirq: Warn when handler enables interrupts
> > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=b738a50a
> > >
> > > wouldn't genirq report a warning on m68k?
> > >
> >
> > 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

> Isn't arch_irqs_disabled() a status reflection of irq disable API?
>

Why not?

Are all interrupts (including NMI) masked whenever arch_irqs_disabled()
returns true on your platforms?

> Thanks
> Barry
>
>