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 05:14:11 +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: Wednesday, February 10, 2021 5:16 PM
> 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: [Linuxarm] Re: [PATCH for-next 00/32] spin lock usage optimization
> for SCSI drivers
> 
> 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 Is the comment in sonic_interrupt() outdated according to this: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, how could spin_lock_irqsave() block the
prioritized interrupts? Isn't arch_irqs_disabled() a status reflection of
irq disable API?

Thanks
Barry