x86: Fix up irq vector map logic

We need to make sure that cfg->used_vector is only cleared once;
otherwise there may be a race condition that allows the same vector to
be assigned twice, defeating the whole purpose of the map.

This makes two changes:
* __clear_irq_vector() only clears the vector if the irq is not being
moved
* smp_iqr_move_cleanup_interrupt() only clears used_vector if this
is the last place it's being used (move_cleanup_count==0 after
decrement).

Also make use of asserts more consistent, to catch this kind of logic
bug in the future.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/xen/arch/x86/io_apic.c b/xen/arch/x86/io_apic.c
index f9565df58b..dd45ff414e 100644
--- a/xen/arch/x86/io_apic.c
+++ b/xen/arch/x86/io_apic.c
@@ -485,12 +485,14 @@ fastcall void smp_irq_move_cleanup_interrupt(struct cpu_user_regs *regs)
                  irq, vector, smp_processor_id());
 
         __get_cpu_var(vector_irq)[vector] = -1;
-        if ( cfg->used_vectors )
+        cfg->move_cleanup_count--;
+
+        if ( cfg->move_cleanup_count == 0 
+             &&  cfg->used_vectors )
         {
             ASSERT(test_bit(vector, cfg->used_vectors));
             clear_bit(vector, cfg->used_vectors);
         }
-        cfg->move_cleanup_count--;
 unlock:
         spin_unlock(&desc->lock);
     }