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