rcu: Call out dangers of expedited RCU primitives

The expedited RCU primitives can be quite useful, but they have some
high costs as well.  This commit updates and creates docbook comments
calling out the costs, and updates the RCU documentation as well.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 16 insertions, 4 deletions

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 07f880445d8..f7ceadf4986 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -835,10 +835,22 @@ sync_rcu_preempt_exp_init(struct rcu_state *rsp, struct rcu_node *rnp)
 		rcu_report_exp_rnp(rsp, rnp, false); /* Don't wake self. */
 }
 
-/*
- * Wait for an rcu-preempt grace period, but expedite it.  The basic idea
- * is to invoke synchronize_sched_expedited() to push all the tasks to
- * the ->blkd_tasks lists and wait for this list to drain.
+/**
+ * synchronize_rcu_expedited - Brute-force RCU grace period
+ *
+ * Wait for an RCU-preempt grace period, but expedite it.  The basic
+ * idea is to invoke synchronize_sched_expedited() to push all the tasks to
+ * the ->blkd_tasks lists and wait for this list to drain.  This consumes
+ * significant time on all CPUs and is unfriendly to real-time workloads,
+ * so is thus not recommended for any sort of common-case code.
+ * In fact, if you are using synchronize_rcu_expedited() in a loop,
+ * please restructure your code to batch your updates, and then Use a
+ * single synchronize_rcu() instead.
+ *
+ * Note that it is illegal to call this function while holding any lock
+ * that is acquired by a CPU-hotplug notifier.  And yes, it is also illegal
+ * to call this function from a CPU-hotplug notifier.  Failing to observe
+ * these restriction will result in deadlock.
  */
 void synchronize_rcu_expedited(void)
 {