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

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Date:   Wed, 10 Feb 2021 21:28:53 +0000
From:   "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>
To:     Finn Thain <fthain@...egraphics.com.au>
CC:     tanxiaofei <tanxiaofei@...wei.com>,
        "jejb@...ux.ibm.com" <jejb@...ux.ibm.com>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxarm@...neuler.org" <linuxarm@...neuler.org>,
        "linux-m68k@...r.kernel.org" <linux-m68k@...r.kernel.org>
Subject: RE: [Linuxarm] Re: [PATCH for-next 00/32] spin lock usage
 optimization for SCSI drivers



> -----Original Message-----
> From: Finn Thain [mailto:fthain@...egraphics.com.au]
> Sent: Thursday, February 11, 2021 10:07 AM
> To: Song Bao Hua (Barry Song) <song.bao.hua@...ilicon.com>
> Cc: tanxiaofei <tanxiaofei@...wei.com>; jejb@...ux.ibm.com;
> martin.petersen@...cle.com; linux-scsi@...r.kernel.org;
> linux-kernel@...r.kernel.org; linuxarm@...neuler.org;
> linux-m68k@...r.kernel.org
> Subject: RE: [Linuxarm] Re: [PATCH for-next 00/32] spin lock usage optimization
> for SCSI drivers
> 
> 
> 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 Is the comment in sonic_interrupt() outdated according to this: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

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.

> 
> > 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.

> 
> 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/

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.

> 
> > Thanks
> > Barry
> >

Thanks
Barry