srcu: Use rcu_segcblist to track SRCU callbacks

This commit switches SRCU from custom-built callback queues to the new
rcu_segcblist structure.  This change associates grace-period sequence
numbers with groups of callbacks, which will be needed for efficient
processing of per-CPU callbacks.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

include/linux/rcu_segcblist.h

@@ -92,7 +92,6 @@ static inline struct rcu_head *rcu_cblist_dequeue(struct rcu_cblist *rclp)
 	rhp = rclp->head;
 	if (!rhp)
 		return NULL;
-	prefetch(rhp);
 	rclp->len--;
 	rclp->head = rhp->next;
 	if (!rclp->head)
@@ -175,6 +174,15 @@ struct rcu_segcblist {
 	long len_lazy;
 };
 
+#define RCU_SEGCBLIST_INITIALIZER(n) \
+{ \
+	.head = NULL, \
+	.tails[RCU_DONE_TAIL] = &n.head, \
+	.tails[RCU_WAIT_TAIL] = &n.head, \
+	.tails[RCU_NEXT_READY_TAIL] = &n.head, \
+	.tails[RCU_NEXT_TAIL] = &n.head, \
+}
+
 /*
  * Initialize an rcu_segcblist structure.
  */

include/linux/srcu.h

@@ -22,7 +22,7 @@
  *	   Lai Jiangshan <laijs@cn.fujitsu.com>
  *
  * For detailed explanation of Read-Copy Update mechanism see -
- * 		Documentation/RCU/ *.txt
+ *		Documentation/RCU/ *.txt
  *
  */
 
@@ -32,31 +32,20 @@
 #include <linux/mutex.h>
 #include <linux/rcupdate.h>
 #include <linux/workqueue.h>
+#include <linux/rcu_segcblist.h>
 
 struct srcu_array {
 	unsigned long lock_count[2];
 	unsigned long unlock_count[2];
 };
 
-struct rcu_batch {
-	struct rcu_head *head, **tail;
-};
-
-#define RCU_BATCH_INIT(name) { NULL, &(name.head) }
-
 struct srcu_struct {
 	unsigned long completed;
 	unsigned long srcu_gp_seq;
 	struct srcu_array __percpu *per_cpu_ref;
-	spinlock_t queue_lock; /* protect ->batch_queue, ->running */
+	spinlock_t queue_lock; /* protect ->srcu_cblist, ->srcu_state */
 	int srcu_state;
-	/* callbacks just queued */
-	struct rcu_batch batch_queue;
-	/* callbacks try to do the first check_zero */
-	struct rcu_batch batch_check0;
-	/* callbacks done with the first check_zero and the flip */
-	struct rcu_batch batch_check1;
-	struct rcu_batch batch_done;
+	struct rcu_segcblist srcu_cblist;
 	struct delayed_work work;
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 	struct lockdep_map dep_map;
@@ -97,10 +86,7 @@ void process_srcu(struct work_struct *work);
 		.per_cpu_ref = &name##_srcu_array,			\
 		.queue_lock = __SPIN_LOCK_UNLOCKED(name.queue_lock),	\
 		.srcu_state = SRCU_STATE_IDLE,				\
-		.batch_queue = RCU_BATCH_INIT(name.batch_queue),	\
-		.batch_check0 = RCU_BATCH_INIT(name.batch_check0),	\
-		.batch_check1 = RCU_BATCH_INIT(name.batch_check1),	\
-		.batch_done = RCU_BATCH_INIT(name.batch_done),		\
+		.srcu_cblist = RCU_SEGCBLIST_INITIALIZER(name.srcu_cblist),\
 		.work = __DELAYED_WORK_INITIALIZER(name.work, process_srcu, 0),\
 		__SRCU_DEP_MAP_INIT(name)				\
 	}

kernel/rcu/rcu.h

@@ -87,6 +87,12 @@ static inline unsigned long rcu_seq_snap(unsigned long *sp)
 	return s;
 }
 
+/* Return the current value the update side's sequence number, no ordering. */
+static inline unsigned long rcu_seq_current(unsigned long *sp)
+{
+	return READ_ONCE(*sp);
+}
+
 /*
  * Given a snapshot from rcu_seq_snap(), determine whether or not a
  * full update-side operation has occurred.

kernel/rcu/srcu.c

@@ -22,7 +22,7 @@
  *	   Lai Jiangshan <laijs@cn.fujitsu.com>
  *
  * For detailed explanation of Read-Copy Update mechanism see -
- * 		Documentation/RCU/ *.txt
+ *		Documentation/RCU/ *.txt
  *
  */
 
@@ -38,85 +38,13 @@
 
 #include "rcu.h"
 
-/*
- * Initialize an rcu_batch structure to empty.
- */
-static inline void rcu_batch_init(struct rcu_batch *b)
-{
-	b->head = NULL;
-	b->tail = &b->head;
-}
-
-/*
- * Enqueue a callback onto the tail of the specified rcu_batch structure.
- */
-static inline void rcu_batch_queue(struct rcu_batch *b, struct rcu_head *head)
-{
-	*b->tail = head;
-	b->tail = &head->next;
-}
-
-/*
- * Is the specified rcu_batch structure empty?
- */
-static inline bool rcu_batch_empty(struct rcu_batch *b)
-{
-	return b->tail == &b->head;
-}
-
-/*
- * Are all batches empty for the specified srcu_struct?
- */
-static inline bool rcu_all_batches_empty(struct srcu_struct *sp)
-{
-	return rcu_batch_empty(&sp->batch_done) &&
-	       rcu_batch_empty(&sp->batch_check1) &&
-	       rcu_batch_empty(&sp->batch_check0) &&
-	       rcu_batch_empty(&sp->batch_queue);
-}
-
-/*
- * Remove the callback at the head of the specified rcu_batch structure
- * and return a pointer to it, or return NULL if the structure is empty.
- */
-static inline struct rcu_head *rcu_batch_dequeue(struct rcu_batch *b)
-{
-	struct rcu_head *head;
-
-	if (rcu_batch_empty(b))
-		return NULL;
-
-	head = b->head;
-	b->head = head->next;
-	if (b->tail == &head->next)
-		rcu_batch_init(b);
-
-	return head;
-}
-
-/*
- * Move all callbacks from the rcu_batch structure specified by "from" to
- * the structure specified by "to".
- */
-static inline void rcu_batch_move(struct rcu_batch *to, struct rcu_batch *from)
-{
-	if (!rcu_batch_empty(from)) {
-		*to->tail = from->head;
-		to->tail = from->tail;
-		rcu_batch_init(from);
-	}
-}
-
 static int init_srcu_struct_fields(struct srcu_struct *sp)
 {
 	sp->completed = 0;
 	sp->srcu_gp_seq = 0;
 	spin_lock_init(&sp->queue_lock);
 	sp->srcu_state = SRCU_STATE_IDLE;
-	rcu_batch_init(&sp->batch_queue);
-	rcu_batch_init(&sp->batch_check0);
-	rcu_batch_init(&sp->batch_check1);
-	rcu_batch_init(&sp->batch_done);
+	rcu_segcblist_init(&sp->srcu_cblist);
 	INIT_DELAYED_WORK(&sp->work, process_srcu);
 	sp->per_cpu_ref = alloc_percpu(struct srcu_array);
 	return sp->per_cpu_ref ? 0 : -ENOMEM;
@@ -268,7 +196,7 @@ void cleanup_srcu_struct(struct srcu_struct *sp)
 {
 	if (WARN_ON(srcu_readers_active(sp)))
 		return; /* Leakage unless caller handles error. */
-	if (WARN_ON(!rcu_all_batches_empty(sp)))
+	if (WARN_ON(!rcu_segcblist_empty(&sp->srcu_cblist)))
 		return; /* Leakage unless caller handles error. */
 	flush_delayed_work(&sp->work);
 	if (WARN_ON(READ_ONCE(sp->srcu_state) != SRCU_STATE_IDLE))
@@ -324,6 +252,8 @@ EXPORT_SYMBOL_GPL(__srcu_read_unlock);
  */
 static void srcu_gp_start(struct srcu_struct *sp)
 {
+	rcu_segcblist_accelerate(&sp->srcu_cblist,
+				 rcu_seq_snap(&sp->srcu_gp_seq));
 	WRITE_ONCE(sp->srcu_state, SRCU_STATE_SCAN1);
 	rcu_seq_start(&sp->srcu_gp_seq);
 }
@@ -371,6 +301,11 @@ static void srcu_gp_end(struct srcu_struct *sp)
 {
 	rcu_seq_end(&sp->srcu_gp_seq);
 	WRITE_ONCE(sp->srcu_state, SRCU_STATE_DONE);
+
+	spin_lock_irq(&sp->queue_lock);
+	rcu_segcblist_advance(&sp->srcu_cblist,
+			      rcu_seq_current(&sp->srcu_gp_seq));
+	spin_unlock_irq(&sp->queue_lock);
 }
 
 /*
@@ -409,7 +344,7 @@ void call_srcu(struct srcu_struct *sp, struct rcu_head *head,
 	head->func = func;
 	spin_lock_irqsave(&sp->queue_lock, flags);
 	smp_mb__after_unlock_lock(); /* Caller's prior accesses before GP. */
-	rcu_batch_queue(&sp->batch_queue, head);
+	rcu_segcblist_enqueue(&sp->srcu_cblist, head, false);
 	if (READ_ONCE(sp->srcu_state) == SRCU_STATE_IDLE) {
 		srcu_gp_start(sp);
 		queue_delayed_work(system_power_efficient_wq, &sp->work, 0);
@@ -445,13 +380,13 @@ static void __synchronize_srcu(struct srcu_struct *sp, int trycount)
 	smp_mb__after_unlock_lock(); /* Caller's prior accesses before GP. */
 	if (READ_ONCE(sp->srcu_state) == SRCU_STATE_IDLE) {
 		/* steal the processing owner */
+		rcu_segcblist_enqueue(&sp->srcu_cblist, head, false);
 		srcu_gp_start(sp);
-		rcu_batch_queue(&sp->batch_check0, head);
 		spin_unlock_irq(&sp->queue_lock);
 		/* give the processing owner to work_struct */
 		srcu_reschedule(sp, 0);
 	} else {
-		rcu_batch_queue(&sp->batch_queue, head);
+		rcu_segcblist_enqueue(&sp->srcu_cblist, head, false);
 		spin_unlock_irq(&sp->queue_lock);
 	}
 
@@ -548,19 +483,6 @@ EXPORT_SYMBOL_GPL(srcu_batches_completed);
 #define SRCU_CALLBACK_BATCH	10
 #define SRCU_INTERVAL		1
 
-/*
- * Move any new SRCU callbacks to the first stage of the SRCU grace
- * period pipeline.
- */
-static void srcu_collect_new(struct srcu_struct *sp)
-{
-	if (!rcu_batch_empty(&sp->batch_queue)) {
-		spin_lock_irq(&sp->queue_lock);
-		rcu_batch_move(&sp->batch_check0, &sp->batch_queue);
-		spin_unlock_irq(&sp->queue_lock);
-	}
-}
-
 /*
  * Core SRCU state machine.  Advance callbacks from ->batch_check0 to
  * ->batch_check1 and then to ->batch_done as readers drain.
@@ -586,26 +508,7 @@ static void srcu_advance_batches(struct srcu_struct *sp, int trycount)
 		idx = 1 ^ (sp->completed & 1);
 		if (!try_check_zero(sp, idx, trycount))
 			return; /* readers present, retry after SRCU_INTERVAL */
-
-		/*
-		 * The callbacks in ->batch_check1 have already done
-		 * with their first zero check and flip back when they were
-		 * enqueued on ->batch_check0 in a previous invocation of
-		 * srcu_advance_batches().  (Presumably try_check_zero()
-		 * returned false during that invocation, leaving the
-		 * callbacks stranded on ->batch_check1.) They are therefore
-		 * ready to invoke, so move them to ->batch_done.
-		 */
-		rcu_batch_move(&sp->batch_done, &sp->batch_check1);
 		srcu_flip(sp);
-
-		/*
-		 * The callbacks in ->batch_check0 just finished their
-		 * first check zero and flip, so move them to ->batch_check1
-		 * for future checking on the other idx.
-		 */
-		rcu_batch_move(&sp->batch_check1, &sp->batch_check0);
-
 		WRITE_ONCE(sp->srcu_state, SRCU_STATE_SCAN2);
 	}
 
@@ -619,14 +522,6 @@ static void srcu_advance_batches(struct srcu_struct *sp, int trycount)
 		trycount = trycount < 2 ? 2 : trycount;
 		if (!try_check_zero(sp, idx, trycount))
 			return; /* readers present, retry after SRCU_INTERVAL */
-
-		/*
-		 * The callbacks in ->batch_check1 have now waited for
-		 * all pre-existing readers using both idx values.  They are
-		 * therefore ready to invoke, so move them to ->batch_done.
-		 */
-		rcu_batch_move(&sp->batch_done, &sp->batch_check1);
-
 		srcu_gp_end(sp);
 	}
 }
@@ -639,17 +534,26 @@ static void srcu_advance_batches(struct srcu_struct *sp, int trycount)
  */
 static void srcu_invoke_callbacks(struct srcu_struct *sp)
 {
-	int i;
-	struct rcu_head *head;
+	struct rcu_cblist ready_cbs;
+	struct rcu_head *rhp;
 
-	for (i = 0; i < SRCU_CALLBACK_BATCH; i++) {
-		head = rcu_batch_dequeue(&sp->batch_done);
-		if (!head)
-			break;
+	spin_lock_irq(&sp->queue_lock);
+	if (!rcu_segcblist_ready_cbs(&sp->srcu_cblist)) {
+		spin_unlock_irq(&sp->queue_lock);
+		return;
+	}
+	rcu_cblist_init(&ready_cbs);
+	rcu_segcblist_extract_done_cbs(&sp->srcu_cblist, &ready_cbs);
+	spin_unlock_irq(&sp->queue_lock);
+	rhp = rcu_cblist_dequeue(&ready_cbs);
+	for (; rhp != NULL; rhp = rcu_cblist_dequeue(&ready_cbs)) {
 		local_bh_disable();
-		head->func(head);
+		rhp->func(rhp);
 		local_bh_enable();
 	}
+	spin_lock_irq(&sp->queue_lock);
+	rcu_segcblist_insert_count(&sp->srcu_cblist, &ready_cbs);
+	spin_unlock_irq(&sp->queue_lock);
 }
 
 /*
@@ -660,9 +564,9 @@ static void srcu_reschedule(struct srcu_struct *sp, unsigned long delay)
 {
 	bool pending = true;
 
-	if (rcu_all_batches_empty(sp)) {
+	if (rcu_segcblist_empty(&sp->srcu_cblist)) {
 		spin_lock_irq(&sp->queue_lock);
-		if (rcu_all_batches_empty(sp) &&
+		if (rcu_segcblist_empty(&sp->srcu_cblist) &&
 		    READ_ONCE(sp->srcu_state) == SRCU_STATE_DONE) {
 			WRITE_ONCE(sp->srcu_state, SRCU_STATE_IDLE);
 			pending = false;
@@ -683,7 +587,6 @@ void process_srcu(struct work_struct *work)
 
 	sp = container_of(work, struct srcu_struct, work.work);
 
-	srcu_collect_new(sp);
 	srcu_advance_batches(sp, 1);
 	srcu_invoke_callbacks(sp);
 	srcu_reschedule(sp, SRCU_INTERVAL);

kernel/rcu/tree.h

@@ -30,7 +30,7 @@
 #include <linux/seqlock.h>
 #include <linux/swait.h>
 #include <linux/stop_machine.h>
-#include "rcu_segcblist.h"
+#include <linux/rcu_segcblist.h>
 
 /*
  * Define shape of hierarchy based on NR_CPUS, CONFIG_RCU_FANOUT, and