Ingo,
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
tracing.
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))
return;
if (unlikely(desc->status & IRQ_INPROGRESS))
+ return;
+
desc->chip->mask(irq);
move_masked_irq(irq);
desc->chip->unmask(irq);
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 <[email protected]>
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)))
return;
@@ -68,8 +69,17 @@ void move_native_irq(int irq)
if (unlikely(desc->status & IRQ_DISABLED))
return;
- 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);
move_masked_irq(irq);
- desc->chip->unmask(irq);
+ if (mask)
+ desc->chip->unmask(irq);
}
-
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