1 files changed, 16 insertions, 2 deletions

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 643c8f650dd..c22c4ef2a0d 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -1041,6 +1041,8 @@ static int synchronize_sched_expedited_cpu_stop(void *data)
 	 * robustness against future implementation changes.
 	 */
 	smp_mb(); /* See above comment block. */
+	if (cpumask_first(cpu_online_mask) == smp_processor_id())
+		atomic_inc(&synchronize_sched_expedited_count);
 	return 0;
 }
 
@@ -1053,13 +1055,26 @@ static int synchronize_sched_expedited_cpu_stop(void *data)
  * Note that it is illegal to call this function while holding any
  * lock that is acquired by a CPU-hotplug notifier.  Failing to
  * observe this restriction will result in deadlock.
+ *
+ * The synchronize_sched_expedited_cpu_stop() function is called
+ * in stop-CPU context, but in order to keep overhead down to a dull
+ * roar, we don't force this function to wait for its counterparts
+ * on other CPUs.  One instance of this function will increment the
+ * synchronize_sched_expedited_count variable per call to
+ * try_stop_cpus(), but there is no guarantee what order this instance
+ * will occur in.  The worst case is that it is last on one call
+ * to try_stop_cpus(), and the first on the next call.  This means
+ * that piggybacking requires that synchronize_sched_expedited_count
+ * be incremented by 3: this guarantees that the piggybacking
+ * task has waited through an entire cycle of context switches,
+ * even in the worst case.
  */
 void synchronize_sched_expedited(void)
 {
 	int snap, trycount = 0;
 
 	smp_mb();  /* ensure prior mod happens before capturing snap. */
-	snap = atomic_read(&synchronize_sched_expedited_count) + 1;
+	snap = atomic_read(&synchronize_sched_expedited_count) + 2;
 	get_online_cpus();
 	while (try_stop_cpus(cpu_online_mask,
 			     synchronize_sched_expedited_cpu_stop,
@@ -1077,7 +1092,6 @@ void synchronize_sched_expedited(void)
 		}
 		get_online_cpus();
 	}
-	atomic_inc(&synchronize_sched_expedited_count);
 	smp_mb__after_atomic_inc(); /* ensure post-GP actions seen after GP. */
 	put_online_cpus();
 }