[PATCH 2.6.16] Shared interrupts sometimes lost

Neil Brown
2006-04-08T04:11:13Z

Hi,
I've been having enormous fun just lately playing with my new Minitar
wireless cards - based on the RaLink 2560 chip set. I have a
couple of PCI cards that seem to work well, and a PCMCIA that mostly
works, but will lockup the computer under load, which isn't nice :-(

This is using the "legacy" rt2500 driver derived from the vendor
provided driver, rather than the newer rt2x00 which I cannot get to
work at all. These are all part of rt2400.sourceforge.net.

Anyway, it turns out that my lockup problem is actually a problem in
the Linux kernel rather than with the driver (though possibly aspects
of the driver make the situation worse). I have a patch, below,
which fixes my problem for me.

The problem is that after a while, the interrupts generated by the
card stop being handled by Linux, so the ISR routine in the driver
doesn't get called, and progress of all sorts stop. The fact that
this leads to a full system lockup is the fault of the driver, not
Linux. But Linux is definitely implicated in the loss of interrupts.
I can work around the problem using "irqpoll", but that is a bit
heavy handed, and shouldn't be needed.

This is my first real introduction to the IRQ handling code in Linux,
so please forgive any little errors. I'm fairly sure the big picture
is right, partly because the patch helps so much.

To explain what I think is happening, let me start with a very simple
case. A number of PCI devices (this one included) have a number of
events which can trigger an interrupt. The events which are current
are presented as bits in a register, and are ORed together (and
possibly masked by another register) to make the IRQ line.
When 1's are written to any bits in this register, it acknowledges
the event and clears the bit.
A typical code fragment is
events = read_register(INTERRUPTS);
write_register(INTERRUPTS, events);
... handle each 1 bits in events ....

This would normally clear all pending events and cause the interrupt
line to go low (or at least to not be asserted).

However there is room for a race here. If an event occurs between
the read and the write, then this will NOT de-assert the IRQ line.
It will remain asserted throughout.

Now if the IRQ is handled as an edge-triggered line (which I believe
they are in Linux), then losing this race will mean that we don't see
any more interrupts on this line.

This is a race that would be fairly hard to hit, and is easily fixed
by the driver. After handling all of 'events', it should read the
INTERRUPTS register again and see if there is anything more. I tried
this and it didn't fix my problem (though it might have made it
happen less often, I'm not certain).

In my particular case (and probably quite commonly with PCMCIA
cards), the IRQ line is shared:

10: 66178 XT-PIC yenta, yenta, ohci_hcd:usb2, ohci_hcd:usb3, ehci_hcd:usb4, eth0

So now the race is somewhat larger.
If yenta-1 handles its interrupt events and then another event
happens for yenta-1 before eth0 gets around to clearing its
interrupts, the shared IRQ line will remain asserted after all IRQ
handlers have been called, and so another edge will never be seen.

This race - with a shared IRQ line - cannot be handled within any one
driver. There needs to be some sort of end-to-end check.

In particular, the list of 'action' handlers needs to be called
repeatedly until a full pass has been made through all, and none of
them have reported that there was anything to do. Only then can we
be sure that the IRQ line has been de-asserted.

The following patch does that, and "works for me". It included a
kernel-parameter which allows me to disable the fix so I can test
that the fix really makes a difference. It does.

I am presenting this patch for comment at the moment rather than
inclusion. If it is generally acceptable, I'll remove the __setup
stuff and put a proper change-log at the top etc.

It might be appropriate to put a loop limit in so it doesn't more
than 1000 times or something, just in case someone claims to always
handle the interrupt. Is that needed?

Looking forward to your comments,
NeilBrown

------------------------------
Signed-off-by: Neil Brown <neilb@suse.de>

### Diffstat output
./kernel/irq/handle.c | 28 ++++++++++++++++++++++++++--
1 file changed, 26 insertions(+), 2 deletions(-)

diff ./kernel/irq/handle.c~current~ ./kernel/irq/handle.c
--- ./kernel/irq/handle.c~current~ 2006-04-08 13:26:46.000000000 +1000
+++ ./kernel/irq/handle.c 2006-04-08 13:27:05.000000000 +1000
@@ -73,23 +73,47 @@ irqreturn_t no_action(int cpl, void *dev
return IRQ_NONE;
}

+static int safeirq = 1;
+int __init unsafeirq_setup(char *str)
+{
+ safeirq = 0;
+ printk(KERN_INFO "IRQ safety-line removed - good luck\n");
+ return 1;
+}
+__setup("unsafeirq", unsafeirq_setup);
/*
* Have got an event to handle:
*/
fastcall int handle_IRQ_event(unsigned int irq, struct pt_regs *regs,
- struct irqaction *action)
+ struct irqaction *actionlist)
{
int ret, retval = 0, status = 0;
+ struct irqaction *action = actionlist;
+ int repeat=0;

if (!(action->flags & SA_INTERRUPT))
local_irq_enable();

do {
ret = action->handler(irq, action->dev_id, regs);
- if (ret == IRQ_HANDLED)
+ if (ret == IRQ_HANDLED) {
status |= action->flags;
+ repeat = 1;
+ }
retval |= ret;
action = action->next;
+ if (!action &&
+ repeat &&
+ safeirq &&
+ (actionlist->flags & SA_SHIRQ)) {
+ /* at least one handler on the list did something,
+ * and the interrupt is sharable, so give
+ * every handler another chance, incase a new event
+ * came in and is holding the irq line asserted.
+ */
+ action = actionlist;
+ repeat = 0;
+ }
} while (action);

if (status & SA_SAMPLE_RANDOM)