From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S266096AbUHDOTh (ORCPT ); Wed, 4 Aug 2004 10:19:37 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S266075AbUHDOTg (ORCPT ); Wed, 4 Aug 2004 10:19:36 -0400 Received: from bi01p1.co.us.ibm.com ([32.97.110.142]:26220 "EHLO linux.local") by vger.kernel.org with ESMTP id S266069AbUHDOTZ (ORCPT ); Wed, 4 Aug 2004 10:19:25 -0400 Date: Wed, 4 Aug 2004 07:16:08 -0700 From: "Paul E. McKenney" To: dipankar@in.ibm.com, shemminger@osdl.org Cc: linux-kernel@vger.kernel.org Subject: [RFC][PATCH] Updated RCU documentation improvement Message-ID: <20040804141608.GA1865@us.ibm.com> Reply-To: paulmck@us.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.4.1i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Hello! 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. Thoughts? Thanx, Paul diff -urpN -X ../dontdiff linux-2.5/include/linux/rcupdate.h linux-2.5.rcu_read_lock_comments/include/linux/rcupdate.h --- linux-2.5/include/linux/rcupdate.h Mon Aug 2 11:32:10 2004 +++ linux-2.5.rcu_read_lock_comments/include/linux/rcupdate.h Wed Aug 4 06:32:07 2004 @@ -133,8 +133,53 @@ 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-2.5/kernel/rcupdate.c linux-2.5.rcu_read_lock_comments/kernel/rcupdate.c --- linux-2.5/kernel/rcupdate.c Mon Aug 2 11:32:13 2004 +++ linux-2.5.rcu_read_lock_comments/kernel/rcupdate.c Wed Aug 4 06:34:27 2004 @@ -65,14 +65,15 @@ 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 * - * 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)) @@ -370,8 +371,13 @@ 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) {