Messages in this thread Patch in this message | | | Date | Mon, 19 Jul 2004 19:46:29 -0700 | From | Paul Mckenney <> | Subject | [RFC][PATCH] More comment improvements for RCU primitives |
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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) {
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