commit 94777fc51b3ad85ff9f705ddf7cdd0eb3bbad5a6
Author: Dimitri Sivanich <sivanich@sgi.com>
Date:   Tue Oct 16 07:50:21 2012 -0500

    x86/irq/ioapic: Check for valid irq_cfg pointer in smp_irq_move_cleanup_interrupt
    
    Posting this patch to fix an issue concerning sparse irq's that
    I raised a while back.  There was discussion about adding
    refcounting to sparse irqs (to fix other potential race
    conditions), but that does not appear to have been addressed
    yet.  This covers the only issue of this type that I've
    encountered in this area.
    
    A NULL pointer dereference can occur in
    smp_irq_move_cleanup_interrupt() if we haven't yet setup the
    irq_cfg pointer in the irq_desc.irq_data.chip_data.
    
    In create_irq_nr() there is a window where we have set
    vector_irq in __assign_irq_vector(), but not yet called
    irq_set_chip_data() to set the irq_cfg pointer.
    
    Should an IRQ_MOVE_CLEANUP_VECTOR hit the cpu in question during
    this time, smp_irq_move_cleanup_interrupt() will attempt to
    process the aforementioned irq, but panic when accessing
    irq_cfg.
    
    Only continue processing the irq if irq_cfg is non-NULL.
    
    Signed-off-by: Dimitri Sivanich <sivanich@sgi.com>
    Cc: Suresh Siddha <suresh.b.siddha@intel.com>
    Cc: Joerg Roedel <joerg.roedel@amd.com>
    Cc: Yinghai Lu <yinghai@kernel.org>
    Cc: Alexander Gordeev <agordeev@redhat.com>
    Link: http://lkml.kernel.org/r/20121016125021.GA22935@sgi.com
    Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/kernel/apic/io_apic.c b/arch/x86/kernel/apic/io_apic.c
index c265593..1817fa9 100644
--- a/arch/x86/kernel/apic/io_apic.c
+++ b/arch/x86/kernel/apic/io_apic.c
@@ -2257,6 +2257,9 @@ asmlinkage void smp_irq_move_cleanup_interrupt(void)
 			continue;

 		cfg = irq_cfg(irq);
+		if (!cfg)
+			continue;
+
 		raw_spin_lock(&desc->lock);

 		/*