From: Dipankar Sarma 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 Signed-off-by: Andrew Morton --- 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) { _