rcu: Improve comments describing RCU read-side critical sections

There are a number of places that call out the fact that preempt-disable
regions of code now act as RCU read-side critical sections, where
preempt-disable regions of code include irq-disable regions of code,
bh-disable regions of code, hardirq handlers, and NMI handlers.  However,
someone relying solely on (for example) the call_rcu() header comment
might well have no idea that preempt-disable regions of code have RCU
semantics.

This commit therefore updates the header comments for
call_rcu(), synchronize_rcu(), rcu_dereference_bh_check(), and
rcu_dereference_sched_check() to call out these new(ish) forms of RCU
readers.

Reported-by: Michel Lespinasse <michel@lespinasse.org>
[ paulmck: Apply Matthew Wilcox and Michel Lespinasse feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

1 files changed, 14 insertions, 10 deletions

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 2437960a2795..4b00e4fbfa10 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3059,12 +3059,14 @@ __call_rcu(struct rcu_head *head, rcu_callback_t func)
  * period elapses, in other words after all pre-existing RCU read-side
  * critical sections have completed.  However, the callback function
  * might well execute concurrently with RCU read-side critical sections
- * that started after call_rcu() was invoked.  RCU read-side critical
- * sections are delimited by rcu_read_lock() and rcu_read_unlock(), and
- * may be nested.  In addition, regions of code across which interrupts,
- * preemption, or softirqs have been disabled also serve as RCU read-side
- * critical sections.  This includes hardware interrupt handlers, softirq
- * handlers, and NMI handlers.
+ * that started after call_rcu() was invoked.
+ *
+ * RCU read-side critical sections are delimited by rcu_read_lock()
+ * and rcu_read_unlock(), and may be nested.  In addition, but only in
+ * v5.0 and later, regions of code across which interrupts, preemption,
+ * or softirqs have been disabled also serve as RCU read-side critical
+ * sections.  This includes hardware interrupt handlers, softirq handlers,
+ * and NMI handlers.
  *
  * Note that all CPUs must agree that the grace period extended beyond
  * all pre-existing RCU read-side critical section.  On systems with more
@@ -3730,10 +3732,12 @@ static int rcu_blocking_is_gp(void)
  * read-side critical sections have completed.  Note, however, that
  * upon return from synchronize_rcu(), the caller might well be executing
  * concurrently with new RCU read-side critical sections that began while
- * synchronize_rcu() was waiting.  RCU read-side critical sections are
- * delimited by rcu_read_lock() and rcu_read_unlock(), and may be nested.
- * In addition, regions of code across which interrupts, preemption, or
- * softirqs have been disabled also serve as RCU read-side critical
+ * synchronize_rcu() was waiting.
+ *
+ * RCU read-side critical sections are delimited by rcu_read_lock()
+ * and rcu_read_unlock(), and may be nested.  In addition, but only in
+ * v5.0 and later, regions of code across which interrupts, preemption,
+ * or softirqs have been disabled also serve as RCU read-side critical
  * sections.  This includes hardware interrupt handlers, softirq handlers,
  * and NMI handlers.
  *