1 files changed, 26 insertions, 4 deletions

diff --git a/arch/x86/lib/delay_64.c b/arch/x86/lib/delay_64.c
index bbc610518516..4c441be92641 100644
--- a/arch/x86/lib/delay_64.c
+++ b/arch/x86/lib/delay_64.c
@@ -31,14 +31,36 @@ int __devinit read_current_timer(unsigned long *timer_value)
 void __delay(unsigned long loops)
 {
         unsigned bclock, now;
+        int cpu;
 
-        preempt_disable();              /* TSC's are pre-cpu */
+        preempt_disable();
+        cpu = smp_processor_id();
         rdtscl(bclock);
-        do {
-                rep_nop(); 
+        for (;;) {
                 rdtscl(now);
+                if ((now - bclock) >= loops)
+                        break;
+
+                /* Allow RT tasks to run */
+                preempt_enable();
+                rep_nop();
+                preempt_disable();
+
+                /*
+                 * It is possible that we moved to another CPU, and
+                 * since TSC's are per-cpu we need to calculate
+                 * that. The delay must guarantee that we wait "at
+                 * least" the amount of time. Being moved to another
+                 * CPU could make the wait longer but we just need to
+                 * make sure we waited long enough. Rebalance the
+                 * counter for this CPU.
+                 */
+                if (unlikely(cpu != smp_processor_id())) {
+                        loops -= (now - bclock);
+                        cpu = smp_processor_id();
+                        rdtscl(bclock);
+                }
         }
-        while ((now-bclock) < loops);
         preempt_enable();
 }
 EXPORT_SYMBOL(__delay);