-----Original Message-----
From: Finn Thain [mailto:fthain@xxxxxxxxxxxxxxxxxxx]
Sent: Wednesday, February 10, 2021 1:29 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:
On Tue, 9 Feb 2021, Song Bao Hua (Barry Song) wrote:
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.
Sorry for I didn't realize xiaofei had replied.
I was referring to the claim in patch 00/32, i.e. that interrupt handlers
only run when irqs are disabled.
If you put your 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.
It's true that IRQF_DISABLED could be used to avoid the need for irq locks
in interrupt handlers. So, if you want to remove irq locks from interrupt
handlers, today you can't use IRQF_DISABLED to help you. So what?
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.
Prioritized interrupts are beneficial. Why would you want to avoid them?
I doubt this is true as it has been already thought as unnecessary
in Linux:
https://lwn.net/Articles/380931/
Moreover, there's no reason to believe that m68k is the only platform that
supports nested interrupts.
I doubt that is true as genirq is running understand the consumption
that hardIRQ is running in irq-disabled context:
"We run all handlers with interrupts disabled and expect them not to
enable them. Warn when we catch one who does."
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=b738a50a
If it is, m68k is against the assumption of genirq.
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
