rcu: Remove obsolete reference to synchronize_kernel()

The synchronize_kernel() primitive was removed in favor of
synchronize_sched() more than a decade ago, and it seems likely that
rather few kernel hackers are familiar with it.  Its continued presence
is therefore providing more confusion than enlightenment.  This commit
therefore removes the reference from the synchronize_sched() header
comment, and adds the corresponding information to the synchronize_rcu(0
header comment.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 7 insertions, 2 deletions

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index a421753e8e9c..3b432fa4c45b 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -664,8 +664,13 @@ EXPORT_SYMBOL_GPL(call_rcu);
  * synchronize_rcu() was waiting.  RCU read-side critical sections are
  * delimited by rcu_read_lock() and rcu_read_unlock(), and may be nested.
  *
- * See the description of synchronize_sched() for more detailed information
- * on memory ordering guarantees.
+ * See the description of synchronize_sched() for more detailed
+ * information on memory-ordering guarantees.  However, please note
+ * that -only- the memory-ordering guarantees apply.  For example,
+ * synchronize_rcu() is -not- guaranteed to wait on things like code
+ * protected by preempt_disable(), instead, synchronize_rcu() is -only-
+ * guaranteed to wait on RCU read-side critical sections, that is, sections
+ * of code protected by rcu_read_lock().
  */
 void synchronize_rcu(void)
 {