[PATCH][KDUMP] Ignore spurious IPI

[Takao Indoh <indou.takao@jp.fujitsu.com>]

Hi all,

I found a problem that kdump(2nd kernel) sometimes hangs up. It seems
that system panic occurs as follows.

(1)
2nd kernel boot up

(2)
A pending IPI from 1st kernel comes after unmasking interrupts at the
following point.

asmlinkage void __init start_kernel(void)
{
(snip)
    time_init();
    profile_init();
    if (!irqs_disabled())
            printk(KERN_CRIT "start_kernel(): bug: interrupts were "
                             "enabled early\n");
    early_boot_irqs_disabled = false;
    local_irq_enable(); <=======================================HERE

(3)
Kernel tries to handle the interrupt, but some data structures are not
initialized yet at this point. As a result, in the
generic_smp_call_function_single_interrupt(), NULL pointer dereference
occurs when list_replace_init() tries to access &q->list.next.

I took a look at local_apic_timer_interrupt() and found a few lines to
handle such a pending LAPIC interrupt(in this case, timer interrupt).
Therefore I made a patch to ignore spurious IPI in the same manner. I
confirmed this problem does not occur with this patch.

Any comments?

Signed-off-by: Takao Indoh <indou.takao@jp.fujitsu.com>
---
 kernel/smp.c |    6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/smp.c b/kernel/smp.c
index 9910744..f2f561b 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -260,6 +260,12 @@ void generic_smp_call_function_single_interrupt(void)
 	 */
 	WARN_ON_ONCE(!cpu_online(smp_processor_id()));
 
+	if (unlikely(!q->list.next)) {
+		/* Pending interrupt from previous kernel(e.g. kdump), just ignore */
+		pr_warning("Spurious IPI on cpu %d\n", smp_processor_id());
+		return;
+	}
+
 	raw_spin_lock(&q->lock);
 	list_replace_init(&q->list, &list);
 	raw_spin_unlock(&q->lock);