[patch] __do_IRQ does not check IRQ_DISABLED when IRQ_PER_CPU is set

In __do_IRQ(), the normal case is that IRQ_DISABLED is checked and if
set the handler (handle_IRQ_event()) is not called.  

Earlier in __do_IRQ(), if IRQ_PER_CPU is set the code does not check
IRQ_DISABLED and calls the handler even though IRQ_DISABLED is set.
This behavior seems unintentional.

One user encountering this behavior is the CPE handler (in 
arch/ia64/kernel/mca.c).  When the CPE handler encounters too many
CPEs (such as a solid single bit error), it sets up a polling timer
and disables the CPE interrupt (to avoid excessive overhead logging
the stream of single bit errors).  disable_irq_nosync() is called
which sets IRQ_DISABLED.  The IRQ_PER_CPU flag was previously set
(in ia64_mca_late_init()).  The net result is the CPE handler gets
called even though it is marked disabled.

If the behavior of not checking IRQ_DISABLED when IRQ_PER_CPU is
set is intentional, it would be worthy of a comment describing 
the intended behavior.  disable_irq_nosync() does call chip->disable()
to provide a chipset specifiec interface for disabling the interrupt,
which avoids this issue when used.

Comments???

Signed-off-by: Russ Anderson (rja@sgi.com)

--------------------------------------------------------------------
---
 kernel/irq/handle.c |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

Index: linus/kernel/irq/handle.c
===================================================================
--- linus.orig/kernel/irq/handle.c	2007-10-30 09:49:26.000000000 -0500
+++ linus/kernel/irq/handle.c	2007-10-30 10:23:52.436719688 -0500
@@ -178,9 +178,11 @@ fastcall unsigned int __do_IRQ(unsigned 
 		 */
 		if (desc->chip->ack)
 			desc->chip->ack(irq);
-		action_ret = handle_IRQ_event(irq, desc->action);
-		if (!noirqdebug)
-			note_interrupt(irq, desc, action_ret);
+		if (likely(!(desc->status & IRQ_DISABLED))) {
+			action_ret = handle_IRQ_event(irq, desc->action);
+			if ...
[ message continues ]

On Tue, 30 Oct 2007 11:26:57 -0500


Alas, I can't remember who wrote (and cares about) the IRQ_PER_CPU support.
 Oh well.

-

[ message continues ]

Presumably we are in this situation because there are still some
pending CPE interrupts on some cpus when we disable CPE?  Or is
there a more serious problem that we aren't manage to disable CPE
on all cpus properly?

-Tony
-

[ message continues ]

The latter.   If IRQ_PER_CPU is set, IRQ_DISABLED is not checked
in __do_IRQ(), so the handler is always called.  It is not a race
condition type thing where a few pended interrupts get handled after
IRQ_DISABLED is set.

My assumption is that setting IRQ_PER_CPU should not change the
behavior of IRQ_DISABLED.

disable_irq_nosync() does call chip->disable() to provide a chipset
specific interface for disabling the interrupt.  That avoids
the issue by having the chipset not issue the interrupt.  If a 
disable handler is required to disable the interrupt, then setting
IRQ_DISABLED is not necessary (and misleading).  

I think the intended behavior is for chip->disable() to 
disable the interrupt in the chipset.  If, for some reason,
the interrupt cannot be disabled in the hardware, the IRQ_DISABLED
would prevent the interrupt handler from being called.

-- 
Russ Anderson, OS RAS/Partitioning Project Lead  
SGI - Silicon Graphics Inc          rja@sgi.com
-

[ message continues ]