Re: [RFC][PATCH] kdump: x86_64 timer interrupt lockup due to pending interrupt

[Vivek Goyal <vgoyal@in.ibm.com>]

On Tue, Mar 07, 2006 at 04:43:07PM -0700, Eric W. Biederman wrote:
> Vivek Goyal <vgoyal@in.ibm.com> writes:
> > On Mon, Mar 06, 2006 at 10:43:32PM +0100, Andi Kleen wrote:
> >> On Mon, Mar 06, 2006 at 11:40:34AM -0500, Vivek Goyal wrote:
> >> > 

[..]
> >> > 
> >> > o In this patch, one extra EOI is being issued in check_timer() to unlock
> > the
> >> >   vector. Please suggest if there is a better way to handle this situation.
> >> 
> >> Shouldn't we rather do this for all interrupts when the APIC is set up? 
> >> I don't see how the timer is special here.
> >>
> >
> > Timer is a special case here.
> >
> > In other cases, the moment interrupts are enabled on cpu, LAPIC pushes pending
> > interrupts to cpu and it is ignored as bad irq using ack_bad_irq(). This
> > still sends EOI to LAPIC if LPAIC support is compiled in.
> >
> > But for timer, the moment pending interrupt is pushed to cpu, it is handled
> > as valid interrupt and cpu assumes that it came from 8259 and sends ack to
> > 8259 and not to LAPIC. Hence leads to missing EOI for timer vector and 
> > deadlock.
> >
> > But still doing it generic manner for all interrupts while setting up LAPIC
> > probably makes more sense. Please find attached the patch.
> 
> A couple of questions. 
> 
> Does this need to be in #ifdef CONFIG_CRASS_DUMP?
> If this code is truly safe I expect we could run it on every bootup
> simply to be more robust.
> 

AFAIK, we can run this code safely on every bootup and can get rid of
CONFIG_CRASH_DUMP. I have simply put it under it because I observed it
only for crashdump scenarios. But removing this should be good as it
protectets agains buggy boards. Modified patch is attached.


> Why is APIC_ISR_NR a hard code?  I think there is an apic register
> that tells the count.
> 

I did not find any such register. Basically ISR is a 256bit register. We
are reading 32 bits at a time, so logically we can view it as 8, 32 bit
registers. I had two options. Either I put a constant number in for()
loop or #define it. I chose later one.

> Does ack_APIC_irq take an argument?  I am confused that we are calling
> ack_APIC_irq() potentially 8*32 times without passing it anything.
> 

It does not take any argument. Whenever a zero is written to EOI register
LAPIC resets one ISR register bit corresponding to highest priority
interrupt. So if all the ISR bits are set, we will call ack_APIC_irq()
8*32 times to reset them all.

Thanks
Vivek



o check_timer() routine fails while second kernel is booting after a crash
  on an opetron box. Problem happens because timer vector (0x31) seems to be
  locked.

o After a system crash, it is not safe to service interrupts any more, hence
  interrupts are disabled. This leads to pending interrupts at LAPIC. LAPIC
  sends these interrupts to the CPU during early boot of second kernel. Other
  pending interrupts are discarded saying unexpected trap but timer interrupt
  is serviced and CPU does not issue an LAPIC EOI because it think this
  interrupt came from i8259 and sends ack to 8259. This leads to vector 0x31
  locking as LAPIC does not clear respective ISR and keeps on waiting for
  EOI.

o This patch issues extra EOI for the pending interrupts who have ISR set.

o Though today only timer seems to be the special case because in early
  boot it thinks interrupts are coming from i8259 and uses
  mask_and_ack_8259A() as ack handler and does not issue LAPIC EOI. But
  probably doing it in generic manner for all vectors makes sense.

Signed-off-by: Vivek Goyal <vgoyal@in.ibm.com>
---


diff -puN arch/x86_64/kernel/apic.c~x86_64-pending-interrupt-fix arch/x86_64/kernel/apic.c
--- linux-2.6.16-rc5-16M/arch/x86_64/kernel/apic.c~x86_64-pending-interrupt-fix	2006-03-08 11:42:33.000000000 +0530
+++ linux-2.6.16-rc5-16M-root/arch/x86_64/kernel/apic.c	2006-03-08 11:44:49.000000000 +0530
@@ -342,6 +342,7 @@ void __init init_bsp_APIC(void)
 void __cpuinit setup_local_APIC (void)
 {
 	unsigned int value, maxlvt;
+	int i, j;
 
 	value = apic_read(APIC_LVR);
 
@@ -371,6 +372,25 @@ void __cpuinit setup_local_APIC (void)
 	apic_write(APIC_TASKPRI, value);
 
 	/*
+	 * After a crash, we no longer service the interrupts and a pending
+	 * interrupt from previous kernel might still have ISR bit set.
+	 *
+	 * Most probably by now CPU has serviced that pending interrupt and
+	 * it might not have done the ack_APIC_irq() because it thought,
+	 * interrupt came from i8259 as ExtInt. LAPIC did not get EOI so it
+	 * does not clear the ISR bit and cpu thinks it has already serivced
+	 * the interrupt. Hence a vector might get locked. It was noticed
+	 * for timer irq (vector 0x31). Issue an extra EOI to clear ISR.
+	 */
+	for (i = APIC_ISR_NR - 1; i >= 0; i--) {
+		value = apic_read(APIC_ISR + i*0x10);
+		for (j = 31; j >= 0; j--) {
+			if (value & (1<<j))
+				ack_APIC_irq();
+		}
+	}
+
+	/*
 	 * Now that we are all set up, enable the APIC
 	 */
 	value = apic_read(APIC_SPIV);
diff -puN include/asm-x86_64/apicdef.h~x86_64-pending-interrupt-fix include/asm-x86_64/apicdef.h
--- linux-2.6.16-rc5-16M/include/asm-x86_64/apicdef.h~x86_64-pending-interrupt-fix	2006-03-08 11:42:33.000000000 +0530
+++ linux-2.6.16-rc5-16M-root/include/asm-x86_64/apicdef.h	2006-03-08 11:42:33.000000000 +0530
@@ -39,6 +39,7 @@
 #define			APIC_SPIV_FOCUS_DISABLED	(1<<9)
 #define			APIC_SPIV_APIC_ENABLED		(1<<8)
 #define		APIC_ISR	0x100
+#define		APIC_ISR_NR	0x8	/* Number of 32 bit ISR registers. */
 #define		APIC_TMR	0x180
 #define 	APIC_IRR	0x200
 #define 	APIC_ESR	0x280
_
-
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