[PATCH -rt] Stop interrupt storm for fasteoi.

From: Steven Rostedt
Date: Thu Apr 26 2007 - 21:29:28 EST


I've spent several days banging my head on this bug, and I finally found
it. I originally thought we had a bug with the latency tracer, since it
seemed to only occur when I turned on latency tracing. But I guess it
just changed the timings to cause the bug to happen. Now that I found
where the bug is, I don't know how it ever worked, even without the

When taking an Ethernet interrupt, it was handled by handle_fasteoi_irq.

handle_fasteoi_irq, when the irq is handled by a thread, sets the irq
INPROGRESS and masks the irq.

Before leaving handle_fasteoi_irq, a call to desc->chip->eoi is called.

For the apic, this will call move_native_irq.

In the -rt kernel, move_native_irq masks the irq, moves it, and then
blindly unmasks it. So when interrupts are turned on next, we take an
interrupt storm.

The other handlers besides handle_fasteoi_irq mask the irq regardless of
whether the irq is INPROGRESS. But handle_fasteoi_irq will not mask it,
if the irq is already INPROGRESS, and just returns. So we keep taking
the same interrupt.

So, I change move_native_irq to not mask and unmask if the irq is
currently INPROGRESS. But... I'm not sure if this is ok or not, since I
don't know all the uses to this.

My original patch was just to do a

+@@ -68,6 +68,9 @@ void move_native_irq(int irq)
if (unlikely(desc->status & IRQ_DISABLED))

if (unlikely(desc->status & IRQ_INPROGRESS))
+ return;

But I thought that this might be too big of a hammer to this nail. So I
changed it to the patch below.

Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>

Index: linux-2.6.21-rt1-i386/kernel/irq/migration.c
--- linux-2.6.21-rt1-i386.orig/kernel/irq/migration.c
+++ linux-2.6.21-rt1-i386/kernel/irq/migration.c
@@ -61,6 +61,7 @@ void move_masked_irq(int irq)
void move_native_irq(int irq)
struct irq_desc *desc = irq_desc + irq;
+ int mask = 1;

if (likely(!(desc->status & IRQ_MOVE_PENDING)))
@@ -68,8 +69,17 @@ void move_native_irq(int irq)
if (unlikely(desc->status & IRQ_DISABLED))

- desc->chip->mask(irq);
+ /*
+ * If the irq is already in progress, it should be masked.
+ * If we unmask it, we might cause an interrupt storm on RT.
+ */
+ if (unlikely(desc->status & IRQ_INPROGRESS))
+ mask = 0;
+ if (mask)
+ desc->chip->mask(irq);
- desc->chip->unmask(irq);
+ if (mask)
+ desc->chip->unmask(irq);

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/