From: Dipankar Sarma <dipankar@in.ibm.com>
Patch from Paul for additional documentation of api.
Updated based on feedback, and to apply to 2.6.8-rc3. I will be adding more
detailed documentation to the Documentation directory in a separate patch.
Signed-off-by: Paul McKenney <paulmck@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
---
25-akpm/include/linux/rcupdate.h | 63 +++++++++++++++++++++++++++++++++++++++
25-akpm/kernel/rcupdate.c | 39 ++++++++++++++----------
2 files changed, 87 insertions(+), 15 deletions(-)
diff -puN include/linux/rcupdate.h~rcu-document-rcu-api include/linux/rcupdate.h
--- 25/include/linux/rcupdate.h~rcu-document-rcu-api 2004-08-07 02:07:38.832375416 -0700
+++ 25-akpm/include/linux/rcupdate.h 2004-08-07 02:07:38.837374656 -0700
@@ -154,9 +154,72 @@ static inline int rcu_pending(int cpu)
__rcu_pending(&rcu_bh_ctrlblk, &per_cpu(rcu_bh_data, cpu));
}
+/**
+ * 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.
+ */
+
+/**
+ * rcu_read_lock_bh - mark the beginning of a softirq-only RCU critical section
+ *
+ * This is equivalent of rcu_read_lock(), but to be used when updates
+ * are being done using call_rcu_bh(). Since call_rcu_bh() callbacks
+ * consider completion of a softirq handler to be a quiescent state,
+ * a process in RCU read-side critical section must be protected by
+ * disabling softirqs. Read-side critical sections in interrupt context
+ * can use just rcu_read_lock().
+ *
+ */
#define rcu_read_lock_bh() local_bh_disable()
+
+/*
+ * rcu_read_unlock_bh - marks the end of a softirq-only RCU critical section
+ *
+ * See rcu_read_lock_bh() for more information.
+ */
#define rcu_read_unlock_bh() local_bh_enable()
extern void rcu_init(void);
diff -puN kernel/rcupdate.c~rcu-document-rcu-api kernel/rcupdate.c
--- 25/kernel/rcupdate.c~rcu-document-rcu-api 2004-08-07 02:07:38.834375112 -0700
+++ 25-akpm/kernel/rcupdate.c 2004-08-07 02:07:38.838374504 -0700
@@ -73,14 +73,15 @@ static DEFINE_PER_CPU(struct tasklet_str
static int maxbatch = 10;
/**
- * 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
*
- * 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)(struct rcu_head *rcu))
@@ -98,17 +99,20 @@ void fastcall call_rcu(struct rcu_head *
}
/**
- * call_rcu_bh - Queue an RCU update request for which softirq handler
- * completion is a quiescent state.
+ * call_rcu_bh - Queue an RCU for invocation after a quicker grace period.
* @head: structure to be used for queueing the RCU updates.
* @func: actual update function to be invoked after the grace period
*
- * 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
- * or has exited a softirq handler that it may have been executing.
- * The read-side of critical section that use call_rcu_bh() for updation must
- * be protected by rcu_read_lock_bh()/rcu_read_unlock_bh() if it is
- * in process context.
+ * 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. call_rcu_bh() assumes
+ * that the read-side critical sections end on completion of a softirq
+ * handler. This means that read-side critical sections in process
+ * context must not be interrupted by softirqs. This interface is to be
+ * used when most of the read-side critical sections are in softirq context.
+ * RCU read-side critical sections are delimited by rcu_read_lock() and
+ * rcu_read_unlock(), * if in interrupt context or rcu_read_lock_bh()
+ * and rcu_read_unlock_bh(), if in process context. These may be nested.
*/
void fastcall call_rcu_bh(struct rcu_head *head,
void (*func)(struct rcu_head *rcu))
@@ -439,8 +443,13 @@ static void wakeme_after_rcu(struct rcu_
}
/**
- * 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)
{
_