Safely change next->processor if necessary.

Credit2's shared runqueue means that a vcpu may switch from one
pcpu to another without an explicit migration.  We need to
change v->processor to match.  However, this must be done with the
current v->processor schedule lock held.  To avoid deadlock,
do this after we've released the current processor's schedule lock.

Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>

diff -r 669448fb9d0c xen/common/schedule.c
--- a/xen/common/schedule.c	Wed Jan 13 13:47:16 2010 +0000
+++ b/xen/common/schedule.c	Wed Jan 13 14:15:51 2010 +0000
@@ -305,6 +305,23 @@
     vcpu_wake(v);
 }
 
+/* Safely change v->processor when running on a different cpu sharing the same runqueue */
+static void __vcpu_processor_sync(struct vcpu *next)
+{
+    unsigned long flags;
+    int old_cpu;
+    int this_cpu = smp_processor_id();
+
+    vcpu_schedule_lock_irqsave(next, flags);
+
+    /* Switch to new CPU, then unlock old CPU. */
+    old_cpu = next->processor;
+    next->processor = this_cpu;
+
+    spin_unlock_irqrestore(
+        &per_cpu(schedule_data, old_cpu).schedule_lock, flags);
+}
+
 /*
  * Force a VCPU through a deschedule/reschedule path.
  * For example, using this when setting the periodic timer period means that
@@ -852,6 +869,11 @@
 
     spin_unlock_irq(&sd->schedule_lock);
 
+    /* Safely change v->processor if necessary.  Do this after
+     * releasing this cpu's lock to avoid deadlock. */
+    if ( next->processor != smp_processor_id() )
+        __vcpu_processor_sync(next);
+
     perfc_incr(sched_ctx);
 
     stop_timer(&prev->periodic_timer);