[RFC][PATCH] More comment improvements for RCU primitives

[Paul Mckenney <paulmck@agora.rdrop.com>]

Hello!

Here is a patch to improve the usefulness of the RCU primitives'
documentation.  Again, this probably interacts badly with existing
RCU patches, which I will fix when I incorporate feedback.

						Thanx, Paul


diff -urpN -X dontdiff linux-2.6.7/include/linux/rcupdate.h linux-2.6.7-rcu_read_lock_comments/include/linux/rcupdate.h
--- linux-2.6.7/include/linux/rcupdate.h	Tue Jun 15 22:19:02 2004
+++ linux-2.6.7-rcu_read_lock_comments/include/linux/rcupdate.h	Tue Jul 13 18:36:42 2004
@@ -121,8 +121,53 @@ static inline int rcu_pending(int cpu) 
 		return 0;
 }
 
+/**
+ * rcu_read_lock - mark the beginning of an RCU read-side critical section.
+ *
+ * When synchronize_kernel() is invoked on one CPU while other CPUs
+ * are within RCU read-side critical sections, then the
+ * synchronize_kernel() is guaranteed to block until after all the other
+ * CPUs exit their critical sections.  Similarly, if call_rcu() is invoked
+ * on one CPU while other CPUs are within RCU read-side critical
+ * sections, invocation of the corresponding RCU callback is deferred
+ * until after the all the other CPUs exit their critical sections.
+ *
+ * Note, however, that RCU callbacks are permitted to run concurrently
+ * with RCU read-side critical sections.  One way that this can happen
+ * is via the following sequence of events: (1) CPU 0 enters an RCU
+ * read-side critical section, (2) CPU 1 invokes call_rcu() to register
+ * an RCU callback, (3) CPU 0 exits the RCU read-side critical section,
+ * (4) CPU 2 enters a RCU read-side critical section, (5) the RCU
+ * callback is invoked.  This is legal, because the RCU read-side critical
+ * section that was running concurrently with the call_rcu() (and which
+ * therefore might be referencing something that the corresponding RCU
+ * callback would free up) has completed before the corresponding
+ * RCU callback is invoked.
+ *
+ * RCU read-side critical sections may be nested.  Any deferred actions
+ * will be deferred until the outermost RCU read-side critical section
+ * completes.
+ *
+ * It is illegal to block while in an RCU read-side critical section.
+ */
 #define rcu_read_lock()		preempt_disable()
+
+/**
+ * rcu_read_unlock - marks the end of an RCU read-side critical section.
+ *
+ * See rcu_read_lock() for more information.
+ */
 #define rcu_read_unlock()	preempt_enable()
+
+/*
+ * So where is rcu_write_lock()?  It does not exist, as there is no
+ * way for writers to lock out RCU readers.  This is a feature, not
+ * a bug -- this property is what provides RCU's performance benefits.
+ * Of course, writers must coordinate with each other.  The normal
+ * spinlock primitives work well for this, but any other technique may be
+ * used as well.  RCU does not care how the writers keep out of each
+ * others' way, as long as they do so.
+ */
 
 extern void rcu_init(void);
 extern void rcu_check_callbacks(int cpu, int user);
diff -urpN -X dontdiff linux-2.6.7/kernel/rcupdate.c linux-2.6.7-rcu_read_lock_comments/kernel/rcupdate.c
--- linux-2.6.7/kernel/rcupdate.c	Tue Jun 15 22:18:38 2004
+++ linux-2.6.7-rcu_read_lock_comments/kernel/rcupdate.c	Mon Jul 19 11:24:21 2004
@@ -56,15 +56,16 @@ static DEFINE_PER_CPU(struct tasklet_str
 #define RCU_tasklet(cpu) (per_cpu(rcu_tasklet, cpu))
 
 /**
- * call_rcu - Queue an RCU update request.
+ * call_rcu - Queue an RCU callback for invocation after a grace period.
  * @head: structure to be used for queueing the RCU updates.
  * @func: actual update function to be invoked after the grace period
  * @arg: argument to be passed to the update function
  *
- * The update function will be invoked as soon as all CPUs have performed 
- * a context switch or been seen in the idle loop or in a user process. 
- * The read-side of critical section that use call_rcu() for updation must 
- * be protected by rcu_read_lock()/rcu_read_unlock().
+ * The update function will be invoked some time after a full grace
+ * period elapses, in other words after all currently executing RCU
+ * read-side critical sections have completed.  RCU read-side critical
+ * sections are delimited by rcu_read_lock() and rcu_read_unlock(),
+ * and may be nested.
  */
 void fastcall call_rcu(struct rcu_head *head, void (*func)(void *arg), void *arg)
 {
@@ -310,8 +311,13 @@ static void wakeme_after_rcu(void *compl
 }
 
 /**
- * synchronize-kernel - wait until all the CPUs have gone
- * through a "quiescent" state. It may sleep.
+ * synchronize_kernel - wait until a grace period has elapsed.
+ *
+ * Control will return to the caller some time after a full grace
+ * period has elapsed, in other words after all currently executing RCU
+ * read-side critical sections have completed.  RCU read-side critical
+ * sections are delimited by rcu_read_lock() and rcu_read_unlock(),
+ * and may be nested.
  */
 void synchronize_kernel(void)
 {


 Submitted-by: <paulmck@us.ibm.com> AKA <paulmck@agora.rdrop.com>