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

[Song Bao Hua (Barry Song)]

> -----Original Message-----
> From: Finn Thain [mailto:fthain@xxxxxxxxxxxxxxxxxxx]
> Sent: Tuesday, February 9, 2021 6:06 PM
> To: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>
> Cc: tanxiaofei <tanxiaofei@xxxxxxxxxx>; jejb@xxxxxxxxxxxxx;
> martin.petersen@xxxxxxxxxx; linux-scsi@xxxxxxxxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx; linuxarm@xxxxxxxxxxxxx;
> linux-m68k@xxxxxxxxxxxxxxx
> Subject: RE: [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:
> 
> > > -----Original Message-----
> > > From: Finn Thain [mailto:fthain@xxxxxxxxxxxxxxxxxxx]
> > > Sent: Monday, February 8, 2021 8:57 PM
> > > To: tanxiaofei <tanxiaofei@xxxxxxxxxx>
> > > Cc: jejb@xxxxxxxxxxxxx; martin.petersen@xxxxxxxxxx;
> > > linux-scsi@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> > > linuxarm@xxxxxxxxxxxxx
> > > Subject: [Linuxarm] Re: [PATCH for-next 00/32] spin lock usage optimization
> > > for SCSI drivers
> > >
> > > On Sun, 7 Feb 2021, Xiaofei Tan wrote:
> > >
> > > > Replace spin_lock_irqsave with spin_lock in hard IRQ of SCSI drivers.
> > > > There are no function changes, but may speed up if interrupt happen too
> > > > often.
> > >
> > > This change doesn't necessarily work on platforms that support nested
> > > interrupts.
> > >
> > > Were you able to measure any benefit from this change on some other
> > > platform?
> >
> > I think the code disabling irq in hardIRQ is simply wrong.
> > Since this commit
> >
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/
> ?id=e58aa3d2d0cc
> > genirq: Run irq handlers with interrupts disabled
> >
> > interrupt handlers are definitely running in a irq-disabled context
> > unless irq handlers enable them explicitly in the handler to permit
> > other interrupts.
> >
> 
> Repeating the same claim does not somehow make it true. If you put your

Sorry for I didn't realize xiaofei had replied.

> claim to the test, you'll see that that interrupts are not disabled on
> m68k when interrupt handlers execute.

Sounds like an implementation issue of m68k since IRQF_DISABLED has
been totally removed.

> 
> The Interrupt Priority Level (IPL) can prevent any given irq handler from
> being re-entered, but an irq with a higher priority level may be handled
> during execution of a lower priority irq handler.
> 

We used to have IRQF_DISABLED to support so-called "fast interrupt" to avoid
this. But the concept has been totally removed. That is interesting if m68k
still has this issue.

> 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.

Thanks
Barry