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, 17 insertions, 10 deletions

diff --git a/kernel/srcu.c b/kernel/srcu.c
index 3f99fa0e8ed3..ba35f3a4a1f4 100644
--- a/kernel/srcu.c
+++ b/kernel/srcu.c
@@ -286,19 +286,26 @@ void synchronize_srcu(struct srcu_struct *sp)
 EXPORT_SYMBOL_GPL(synchronize_srcu);
 
 /**
- * synchronize_srcu_expedited - like synchronize_srcu, but less patient
+ * synchronize_srcu_expedited - Brute-force SRCU grace period
  * @sp: srcu_struct with which to synchronize.
  *
- * Flip the completed counter, and wait for the old count to drain to zero.
- * As with classic RCU, the updater must use some separate means of
- * synchronizing concurrent updates.  Can block; must be called from
- * process context.
+ * Wait for an SRCU grace period to elapse, but use a "big hammer"
+ * approach to force the grace period to end quickly.  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_srcu_expedited() in a loop, please
+ * restructure your code to batch your updates, and then use a single
+ * synchronize_srcu() instead.
  *
- * Note that it is illegal to call synchronize_srcu_expedited()
- * from the corresponding SRCU read-side critical section; doing so
- * will result in deadlock.  However, it is perfectly legal to call
- * synchronize_srcu_expedited() on one srcu_struct from some other
- * srcu_struct's read-side critical section.
+ * 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.  It is also illegal to call
+ * synchronize_srcu_expedited() from the corresponding SRCU read-side
+ * critical section; doing so will result in deadlock.  However, it is
+ * perfectly legal to call synchronize_srcu_expedited() on one srcu_struct
+ * from some other srcu_struct's read-side critical section, as long as
+ * the resulting graph of srcu_structs is acyclic.
  */
 void synchronize_srcu_expedited(struct srcu_struct *sp)
 {