mutex: implement adaptive spinning

[deliverable/linux.git] / kernel / sched.c

diff --git a/kernel/sched.c b/kernel/sched.c
index b001c133c359136f0d282a97cbe4989d8980472e..589e7308c6155d9bd1db39c869c425720beb2d5f 100644 (file)
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -4614,6 +4614,67 @@ need_resched:
 }
 EXPORT_SYMBOL(schedule);
 
+#ifdef CONFIG_SMP
+/*
+ * Look out! "owner" is an entirely speculative pointer
+ * access and not reliable.
+ */
+int mutex_spin_on_owner(struct mutex *lock, struct thread_info *owner)
+{
+       unsigned int cpu;
+       struct rq *rq;
+
+       if (!sched_feat(OWNER_SPIN))
+               return 0;
+
+#ifdef CONFIG_DEBUG_PAGEALLOC
+       /*
+        * Need to access the cpu field knowing that
+        * DEBUG_PAGEALLOC could have unmapped it if
+        * the mutex owner just released it and exited.
+        */
+       if (probe_kernel_address(&owner->cpu, cpu))
+               goto out;
+#else
+       cpu = owner->cpu;
+#endif
+
+       /*
+        * Even if the access succeeded (likely case),
+        * the cpu field may no longer be valid.
+        */
+       if (cpu >= nr_cpumask_bits)
+               goto out;
+
+       /*
+        * We need to validate that we can do a
+        * get_cpu() and that we have the percpu area.
+        */
+       if (!cpu_online(cpu))
+               goto out;
+
+       rq = cpu_rq(cpu);
+
+       for (;;) {
+               /*
+                * Owner changed, break to re-assess state.
+                */
+               if (lock->owner != owner)
+                       break;
+
+               /*
+                * Is that owner really running on that cpu?
+                */
+               if (task_thread_info(rq->curr) != owner || need_resched())
+                       return 0;
+
+               cpu_relax();
+       }
+out:
+       return 1;
+}
+#endif
+
 #ifdef CONFIG_PREEMPT
 /*
  * this is the entry point to schedule() from in-kernel preemption