srcu: Abstract multi-tail callback list handling

RCU has only one multi-tail callback list, which is implemented via
the nxtlist, nxttail, nxtcompleted, qlen_lazy, and qlen fields in the
rcu_data structure, and whose operations are open-code throughout the
Tree RCU implementation.  This has been more or less OK in the past,
but upcoming callback-list optimizations in SRCU could really use
a multi-tail callback list there as well.

This commit therefore abstracts the multi-tail callback list handling
into a new kernel/rcu/rcu_segcblist.h file, and uses this new API.
The simple head-and-tail pointer callback list is also abstracted and
applied everywhere except for the NOCB callback-offload lists.  (Yes,
the plan is to apply them there as well, but this commit is already
bigger than would be good.)

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

1 files changed, 25 insertions, 29 deletions

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 621296a6694b..f88356652dcf 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1350,10 +1350,10 @@ static bool __maybe_unused rcu_try_advance_all_cbs(void)
 		 */
 		if ((rdp->completed != rnp->completed ||
 		     unlikely(READ_ONCE(rdp->gpwrap))) &&
-		    rdp->nxttail[RCU_DONE_TAIL] != rdp->nxttail[RCU_NEXT_TAIL])
+		    rcu_segcblist_pend_cbs(&rdp->cblist))
 			note_gp_changes(rsp, rdp);
 
-		if (cpu_has_callbacks_ready_to_invoke(rdp))
+		if (rcu_segcblist_ready_cbs(&rdp->cblist))
 			cbs_ready = true;
 	}
 	return cbs_ready;
@@ -1461,7 +1461,7 @@ static void rcu_prepare_for_idle(void)
 	rdtp->last_accelerate = jiffies;
 	for_each_rcu_flavor(rsp) {
 		rdp = this_cpu_ptr(rsp->rda);
-		if (!*rdp->nxttail[RCU_DONE_TAIL])
+		if (rcu_segcblist_pend_cbs(&rdp->cblist))
 			continue;
 		rnp = rdp->mynode;
 		raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */
@@ -1529,7 +1529,7 @@ static void rcu_oom_notify_cpu(void *unused)
 
 	for_each_rcu_flavor(rsp) {
 		rdp = raw_cpu_ptr(rsp->rda);
-		if (rdp->qlen_lazy != 0) {
+		if (rcu_segcblist_n_lazy_cbs(&rdp->cblist)) {
 			atomic_inc(&oom_callback_count);
 			rsp->call(&rdp->oom_head, rcu_oom_callback);
 		}
@@ -1934,30 +1934,26 @@ static bool __maybe_unused rcu_nocb_adopt_orphan_cbs(struct rcu_state *rsp,
 						     struct rcu_data *rdp,
 						     unsigned long flags)
 {
-	long ql = rsp->qlen;
-	long qll = rsp->qlen_lazy;
+	long ql = rcu_cblist_n_cbs(&rsp->orphan_done);
+	long qll = rcu_cblist_n_lazy_cbs(&rsp->orphan_done);
 
 	/* If this is not a no-CBs CPU, tell the caller to do it the old way. */
 	if (!rcu_is_nocb_cpu(smp_processor_id()))
 		return false;
-	rsp->qlen = 0;
-	rsp->qlen_lazy = 0;
 
 	/* First, enqueue the donelist, if any.  This preserves CB ordering. */
-	if (rsp->orphan_donelist != NULL) {
-		__call_rcu_nocb_enqueue(rdp, rsp->orphan_donelist,
-					rsp->orphan_donetail, ql, qll, flags);
-		ql = qll = 0;
-		rsp->orphan_donelist = NULL;
-		rsp->orphan_donetail = &rsp->orphan_donelist;
+	if (!rcu_cblist_empty(&rsp->orphan_done)) {
+		__call_rcu_nocb_enqueue(rdp, rcu_cblist_head(&rsp->orphan_done),
+					rcu_cblist_tail(&rsp->orphan_done),
+					ql, qll, flags);
 	}
-	if (rsp->orphan_nxtlist != NULL) {
-		__call_rcu_nocb_enqueue(rdp, rsp->orphan_nxtlist,
-					rsp->orphan_nxttail, ql, qll, flags);
-		ql = qll = 0;
-		rsp->orphan_nxtlist = NULL;
-		rsp->orphan_nxttail = &rsp->orphan_nxtlist;
+	if (!rcu_cblist_empty(&rsp->orphan_pend)) {
+		__call_rcu_nocb_enqueue(rdp, rcu_cblist_head(&rsp->orphan_pend),
+					rcu_cblist_tail(&rsp->orphan_pend),
+					ql, qll, flags);
 	}
+	rcu_cblist_init(&rsp->orphan_done);
+	rcu_cblist_init(&rsp->orphan_pend);
 	return true;
 }
 
@@ -2399,16 +2395,16 @@ static bool init_nocb_callback_list(struct rcu_data *rdp)
 		return false;
 
 	/* If there are early-boot callbacks, move them to nocb lists. */
-	if (rdp->nxtlist) {
-		rdp->nocb_head = rdp->nxtlist;
-		rdp->nocb_tail = rdp->nxttail[RCU_NEXT_TAIL];
-		atomic_long_set(&rdp->nocb_q_count, rdp->qlen);
-		atomic_long_set(&rdp->nocb_q_count_lazy, rdp->qlen_lazy);
-		rdp->nxtlist = NULL;
-		rdp->qlen = 0;
-		rdp->qlen_lazy = 0;
+	if (!rcu_segcblist_empty(&rdp->cblist)) {
+		rdp->nocb_head = rcu_segcblist_head(&rdp->cblist);
+		rdp->nocb_tail = rcu_segcblist_tail(&rdp->cblist);
+		atomic_long_set(&rdp->nocb_q_count,
+				rcu_segcblist_n_cbs(&rdp->cblist));
+		atomic_long_set(&rdp->nocb_q_count_lazy,
+				rcu_segcblist_n_lazy_cbs(&rdp->cblist));
+		rcu_segcblist_init(&rdp->cblist);
 	}
-	rdp->nxttail[RCU_NEXT_TAIL] = NULL;
+	rcu_segcblist_disable(&rdp->cblist);
 	return true;
 }