1 files changed, 56 insertions, 2 deletions

diff --git a/arch/x86_64/kernel/io_apic.c b/arch/x86_64/kernel/io_apic.c
index 1c6c6f724573..050141c0602b 100644
--- a/arch/x86_64/kernel/io_apic.c
+++ b/arch/x86_64/kernel/io_apic.c
@@ -152,6 +152,32 @@ static inline void io_apic_modify(unsigned int apic, unsigned int value)
         writel(value, &io_apic->data);
 }
 
+static int io_apic_level_ack_pending(unsigned int irq)
+{
+        struct irq_pin_list *entry;
+        unsigned long flags;
+        int pending = 0;
+
+        spin_lock_irqsave(&ioapic_lock, flags);
+        entry = irq_2_pin + irq;
+        for (;;) {
+                unsigned int reg;
+                int pin;
+
+                pin = entry->pin;
+                if (pin == -1)
+                        break;
+                reg = io_apic_read(entry->apic, 0x10 + pin*2);
+                /* Is the remote IRR bit set? */
+                pending |= (reg >> 14) & 1;
+                if (!entry->next)
+                        break;
+                entry = irq_2_pin + entry->next;
+        }
+        spin_unlock_irqrestore(&ioapic_lock, flags);
+        return pending;
+}
+
 /*
  * Synchronize the IO-APIC and the CPU by doing
  * a dummy read from the IO-APIC
@@ -1418,9 +1444,37 @@ static void ack_apic_level(unsigned int irq)
         ack_APIC_irq();
 
         /* Now we can move and renable the irq */
-        move_masked_irq(irq);
-        if (unlikely(do_unmask_irq))
+        if (unlikely(do_unmask_irq)) {
+                /* Only migrate the irq if the ack has been received.
+                 *
+                 * On rare occasions the broadcast level triggered ack gets
+                 * delayed going to ioapics, and if we reprogram the
+                 * vector while Remote IRR is still set the irq will never
+                 * fire again.
+                 *
+                 * To prevent this scenario we read the Remote IRR bit
+                 * of the ioapic.  This has two effects.
+                 * - On any sane system the read of the ioapic will
+                 *   flush writes (and acks) going to the ioapic from
+                 *   this cpu.
+                 * - We get to see if the ACK has actually been delivered.
+                 *
+                 * Based on failed experiments of reprogramming the
+                 * ioapic entry from outside of irq context starting
+                 * with masking the ioapic entry and then polling until
+                 * Remote IRR was clear before reprogramming the
+                 * ioapic I don't trust the Remote IRR bit to be
+                 * completey accurate.
+                 *
+                 * However there appears to be no other way to plug
+                 * this race, so if the Remote IRR bit is not
+                 * accurate and is causing problems then it is a hardware bug
+                 * and you can go talk to the chipset vendor about it.
+                 */
+                if (!io_apic_level_ack_pending(irq))
+                        move_masked_irq(irq);
                 unmask_IO_APIC_irq(irq);
+        }
 }
 
 static struct irq_chip ioapic_chip __read_mostly = {