srcu: Introduce CLASSIC_SRCU Kconfig option

The TREE_SRCU rewrite is large and a bit on the non-simple side, so
this commit helps reduce risk by allowing the old v4.11 SRCU algorithm
to be selected using a new CLASSIC_SRCU Kconfig option that depends
on RCU_EXPERT.  The default is to use the new TREE_SRCU and TINY_SRCU
algorithms, in order to help get these the testing that they need.
However, if your users do not require the update-side scalability that
is to be provided by TREE_SRCU, select RCU_EXPERT and then CLASSIC_SRCU
to revert back to the old classic SRCU algorithm.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

include/linux/srcu.h

@@ -60,6 +60,8 @@ int init_srcu_struct(struct srcu_struct *sp);
 #include <linux/srcutiny.h>
 #elif defined(CONFIG_TREE_SRCU)
 #include <linux/srcutree.h>
+#elif defined(CONFIG_CLASSIC_SRCU)
+#include <linux/srcuclassic.h>
 #else
 #error "Unknown SRCU implementation specified to kernel configuration"
 #endif

include/linux/srcuclassic.h

+/*
+ * Sleepable Read-Copy Update mechanism for mutual exclusion,
+ *	classic v4.11 variant.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, you can access it online at
+ * http://www.gnu.org/licenses/gpl-2.0.html.
+ *
+ * Copyright (C) IBM Corporation, 2017
+ *
+ * Author: Paul McKenney <paulmck@us.ibm.com>
+ */
+
+#ifndef _LINUX_SRCU_CLASSIC_H
+#define _LINUX_SRCU_CLASSIC_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;
+	struct srcu_array __percpu *per_cpu_ref;
+	spinlock_t queue_lock; /* protect ->batch_queue, ->running */
+	bool running;
+	/* 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 delayed_work work;
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+	struct lockdep_map dep_map;
+#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
+};
+
+void process_srcu(struct work_struct *work);
+
+#define __SRCU_STRUCT_INIT(name)					\
+	{								\
+		.completed = -300,					\
+		.per_cpu_ref = &name##_srcu_array,			\
+		.queue_lock = __SPIN_LOCK_UNLOCKED(name.queue_lock),	\
+		.running = false,					\
+		.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),		\
+		.work = __DELAYED_WORK_INITIALIZER(name.work, process_srcu, 0),\
+		__SRCU_DEP_MAP_INIT(name)				\
+	}
+
+/*
+ * Define and initialize a srcu struct at build time.
+ * Do -not- call init_srcu_struct() nor cleanup_srcu_struct() on it.
+ *
+ * Note that although DEFINE_STATIC_SRCU() hides the name from other
+ * files, the per-CPU variable rules nevertheless require that the
+ * chosen name be globally unique.  These rules also prohibit use of
+ * DEFINE_STATIC_SRCU() within a function.  If these rules are too
+ * restrictive, declare the srcu_struct manually.  For example, in
+ * each file:
+ *
+ *	static struct srcu_struct my_srcu;
+ *
+ * Then, before the first use of each my_srcu, manually initialize it:
+ *
+ *	init_srcu_struct(&my_srcu);
+ *
+ * See include/linux/percpu-defs.h for the rules on per-CPU variables.
+ */
+#define __DEFINE_SRCU(name, is_static)					\
+	static DEFINE_PER_CPU(struct srcu_array, name##_srcu_array);\
+	is_static struct srcu_struct name = __SRCU_STRUCT_INIT(name)
+#define DEFINE_SRCU(name)		__DEFINE_SRCU(name, /* not static */)
+#define DEFINE_STATIC_SRCU(name)	__DEFINE_SRCU(name, static)
+
+void synchronize_srcu_expedited(struct srcu_struct *sp);
+void srcu_barrier(struct srcu_struct *sp);
+unsigned long srcu_batches_completed(struct srcu_struct *sp);
+
+#endif

init/Kconfig

@@ -526,15 +526,32 @@ config SRCU
 	  permits arbitrary sleeping or blocking within RCU read-side critical
 	  sections.
 
+config CLASSIC_SRCU
+	bool "Use v4.11 classic SRCU implementation"
+	default n
+	depends on RCU_EXPERT && SRCU
+	help
+	  This option selects the traditional well-tested classic SRCU
+	  implementation from v4.11, as might be desired for enterprise
+	  Linux distributions.  Without this option, the shiny new
+	  Tiny SRCU and Tree SRCU implementations are used instead.
+	  At some point, it is hoped that Tiny SRCU and Tree SRCU
+	  will accumulate enough test time and confidence to allow
+	  Classic SRCU to be dropped entirely.
+
+	  Say Y if you need a rock-solid SRCU.
+
+	  Say N if you would like help test Tree SRCU.
+
 config TINY_SRCU
 	bool
-	default y if TINY_RCU
+	default y if TINY_RCU && !CLASSIC_SRCU
 	help
 	  This option selects the single-CPU non-preemptible version of SRCU.
 
 config TREE_SRCU
 	bool
-	default y if !TINY_RCU
+	default y if !TINY_RCU && !CLASSIC_SRCU
 	help
 	  This option selects the full-fledged version of SRCU.
 

kernel/rcu/Makefile

@@ -3,7 +3,8 @@
 KCOV_INSTRUMENT := n
 
 obj-y += update.o sync.o
-obj-$(CONFIG_TREE_SRCU) += srcu.o
+obj-$(CONFIG_CLASSIC_SRCU) += srcu.o
+obj-$(CONFIG_TREE_SRCU) += srcutree.o
 obj-$(CONFIG_TINY_SRCU) += srcutiny.o
 obj-$(CONFIG_RCU_TORTURE_TEST) += rcutorture.o
 obj-$(CONFIG_RCU_PERF_TEST) += rcuperf.o

kernel/rcu/rcutorture.c

@@ -562,7 +562,7 @@ static void srcu_torture_stats(void)
 	int __maybe_unused cpu;
 	int idx;
 
-#ifdef CONFIG_TREE_SRCU
+#if defined(CONFIG_TREE_SRCU) || defined(CONFIG_CLASSIC_SRCU)
 	idx = srcu_ctlp->completed & 0x1;
 	pr_alert("%s%s Tree SRCU per-CPU(idx=%d):",
 		 torture_type, TORTURE_FLAG, idx);