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>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 9ce682242e99c66911dda83c7098d11e310af21c..3bee58fc23b169ec4413e4921caa622edca6a1d9 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3280,12 +3280,6 @@ static inline int rcu_blocking_is_gp(void)
  * to have executed a full memory barrier during the execution of
  * synchronize_sched() -- even if CPU A and CPU B are the same CPU (but
  * again only if the system has more than one CPU).
- *
- * This primitive provides the guarantees made by the (now removed)
- * synchronize_kernel() API.  In contrast, synchronize_rcu() only
- * guarantees that rcu_read_lock() sections will have completed.
- * In "classic RCU", these two guarantees happen to be one and
- * the same, but can differ in realtime RCU implementations.
  */
 void synchronize_sched(void)
 {

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index a421753e8e9c0afd0bf5bbfffe07c65116fe61d6..3b432fa4c45bf900fa5ad70eaca397a9a250046d 100644 (file)
--- 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)
 {