xen: Fix schedule()'s grabbing of the schedule lock

Because the location of the lock can change between the time you read
it and the time you grab it, the per-cpu schedule locks need to check
after lock acquisition that the lock location hasn't changed, and
release and re-try if so.  This change was effected throughout the
source code, but one very important place was apparently missed: in
schedule() itself.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Committed-by: Keir Fraser <keir@xen.org>
xen-unstable changeset:   25162:478bec603d3d
xen-unstable date:        Tue Apr 10 10:41:30 2012 +0100

xen/common/schedule.c

@@ -1122,6 +1122,7 @@ static void schedule(void)
     bool_t                tasklet_work_scheduled = 0;
     struct schedule_data *sd;
     struct task_slice     next_slice;
+    int cpu = smp_processor_id();
 
     ASSERT(!in_atomic());
 
@@ -1146,7 +1147,7 @@ static void schedule(void)
         BUG();
     }
 
-    spin_lock_irq(sd->schedule_lock);
+    pcpu_schedule_lock_irq(cpu);
 
     stop_timer(&sd->s_timer);
     
@@ -1163,7 +1164,7 @@ static void schedule(void)
 
     if ( unlikely(prev == next) )
     {
-        spin_unlock_irq(sd->schedule_lock);
+        pcpu_schedule_unlock_irq(cpu);
         trace_continue_running(next);
         return continue_running(prev);
     }
@@ -1201,7 +1202,7 @@ static void schedule(void)
     ASSERT(!next->is_running);
     next->is_running = 1;
 
-    spin_unlock_irq(sd->schedule_lock);
+    pcpu_schedule_unlock_irq(cpu);
 
     perfc_incr(sched_ctx);