[PATCH RFC tip/core/rcu 0/9] Add rcu_sync and implement percpu_rwsem in terms of it

[PATCH RFC tip/core/rcu 0/9] Add rcu_sync and implement percpu_rwsem in terms of it

[Paul E. McKenney]

Hello!

This series implements an rcu_sync primitive and updates percpu_rwsem to
be implemented in terms of it.  This is an updated version of the series
posted by Oleg Nesterov, responding to feedback from Ingo Molnar.  The
patches in this series, all courtesy of Oleg (and some in turn based
on work by Peter Zijlstra), are as follows:

1.	Create rcu_sync infrastructure.

2.	Simplify rcu_sync using new rcu_sync_ops structure.

3.	Add CONFIG_PROVE_RCU checks.

4.	Introduce rcu_sync_dtor().

5.	Make percpu_free_rwsem() after kzalloc() safe.

6.	Make use of the rcu_sync infrastructure for percpu_rwsem.

7.	Fix the percpu_rwsem comments outdated by rcu_sync.

8.	Clean up the lockdep annotations in percpu_down_read().

9.	Change _wait_rcu_gp() to work around GCC bug 67055.

							Thanx, Paul

------------------------------------------------------------------------

 b/include/linux/percpu-rwsem.h  |    3 
 b/include/linux/rcu_sync.h      |  161 +++++++++++++++++++-----
 b/include/linux/rcupdate.h      |   11 -
 b/kernel/locking/percpu-rwsem.c |   85 ++++---------
 b/kernel/rcu/Makefile           |    2 
 b/kernel/rcu/sync.c             |  259 ++++++++++++++++++++++++++++++++++++----
 6 files changed, 401 insertions(+), 120 deletions(-)

[PATCH RFC tip/core/rcu 1/9] rcu: Create rcu_sync infrastructure

[Paul E. McKenney]

From: Oleg Nesterov <oleg@redhat.com>

The rcu_sync infrastructure can be thought of as infrastructure to be
used to implement reader-writer primitives having extremely lightweight
readers during times when there are no writers.  The first use is in
the percpu_rwsem used by the VFS subsystem.

This infrastructure is functionally equivalent to

        struct rcu_sync_struct {
                atomic_t counter;
        };

	/* Check possibility of fast-path read-side operations. */
        static inline bool rcu_sync_is_idle(struct rcu_sync_struct *rss)
        {
                return atomic_read(&rss->counter) == 0;
        }

	/* Tell readers to use slowpaths. */
        static inline void rcu_sync_enter(struct rcu_sync_struct *rss)
        {
                atomic_inc(&rss->counter);
                synchronize_sched();
        }

	/* Allow readers to once again use fastpaths. */
        static inline void rcu_sync_exit(struct rcu_sync_struct *rss)
        {
                synchronize_sched();
                atomic_dec(&rss->counter);
        }

The main difference is that it records the state and only calls
synchronize_sched() if required.  At least some of the calls to
synchronize_sched() will be optimized away when rcu_sync_enter() and
rcu_sync_exit() are invoked repeatedly in quick succession.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 include/linux/rcu_sync.h |  94 +++++++++++++++++++++++++
 kernel/rcu/Makefile      |   2 +-
 kernel/rcu/sync.c        | 175 +++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 270 insertions(+), 1 deletion(-)
 create mode 100644 include/linux/rcu_sync.h
 create mode 100644 kernel/rcu/sync.c

diff --git a/include/linux/rcu_sync.h b/include/linux/rcu_sync.h
new file mode 100644
index 000000000000..cb044df2e21c
--- /dev/null
+++ b/include/linux/rcu_sync.h
@@ -0,0 +1,94 @@
+/*
+ * RCU-based infrastructure for lightweight reader-writer locking
+ *
+ * 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) 2015, Red Hat, Inc.
+ *
+ * Author: Oleg Nesterov <oleg@redhat.com>
+ */
+
+#ifndef _LINUX_RCU_SYNC_H_
+#define _LINUX_RCU_SYNC_H_
+
+#include <linux/wait.h>
+#include <linux/rcupdate.h>
+
+/* Structure to mediate between updaters and fastpath-using readers.  */
+struct rcu_sync {
+	int			gp_state;
+	int			gp_count;
+	wait_queue_head_t	gp_wait;
+
+	int			cb_state;
+	struct rcu_head		cb_head;
+
+	void (*sync)(void);
+	void (*call)(struct rcu_head *, void (*)(struct rcu_head *));
+};
+
+#define ___RCU_SYNC_INIT(name)						\
+	.gp_state = 0,							\
+	.gp_count = 0,							\
+	.gp_wait = __WAIT_QUEUE_HEAD_INITIALIZER(name.gp_wait),		\
+	.cb_state = 0
+
+#define __RCU_SCHED_SYNC_INIT(name) {					\
+	___RCU_SYNC_INIT(name),						\
+	.sync = synchronize_sched,					\
+	.call = call_rcu_sched,						\
+}
+
+#define __RCU_BH_SYNC_INIT(name) {					\
+	___RCU_SYNC_INIT(name),						\
+	.sync = synchronize_rcu_bh,					\
+	.call = call_rcu_bh,						\
+}
+
+#define __RCU_SYNC_INIT(name) {						\
+	___RCU_SYNC_INIT(name),						\
+	.sync = synchronize_rcu,					\
+	.call = call_rcu,						\
+}
+
+#define DEFINE_RCU_SCHED_SYNC(name)					\
+	struct rcu_sync name = __RCU_SCHED_SYNC_INIT(name)
+
+#define DEFINE_RCU_BH_SYNC(name)					\
+	struct rcu_sync name = __RCU_BH_SYNC_INIT(name)
+
+#define DEFINE_RCU_SYNC(name)						\
+	struct rcu_sync name = __RCU_SYNC_INIT(name)
+
+/**
+ * rcu_sync_is_idle() - Are readers permitted to use their fastpaths?
+ * @rsp: Pointer to rcu_sync structure to use for synchronization
+ *
+ * Returns true if readers are permitted to use their fastpaths.
+ * Must be invoked within an RCU read-side critical section whose
+ * flavor matches that of the rcu_sync struture.
+ */
+static inline bool rcu_sync_is_idle(struct rcu_sync *rsp)
+{
+	return !rsp->gp_state; /* GP_IDLE */
+}
+
+enum rcu_sync_type { RCU_SYNC, RCU_SCHED_SYNC, RCU_BH_SYNC };
+
+extern void rcu_sync_init(struct rcu_sync *, enum rcu_sync_type);
+extern void rcu_sync_enter(struct rcu_sync *);
+extern void rcu_sync_exit(struct rcu_sync *);
+
+#endif /* _LINUX_RCU_SYNC_H_ */
diff --git a/kernel/rcu/Makefile b/kernel/rcu/Makefile
index 50a808424b06..61a16569ffbf 100644
--- a/kernel/rcu/Makefile
+++ b/kernel/rcu/Makefile
@@ -1,4 +1,4 @@
-obj-y += update.o
+obj-y += update.o sync.o
 obj-$(CONFIG_SRCU) += srcu.o
 obj-$(CONFIG_RCU_TORTURE_TEST) += rcutorture.o
 obj-$(CONFIG_TREE_RCU) += tree.o
diff --git a/kernel/rcu/sync.c b/kernel/rcu/sync.c
new file mode 100644
index 000000000000..0a11df43be23
--- /dev/null
+++ b/kernel/rcu/sync.c
@@ -0,0 +1,175 @@
+/*
+ * RCU-based infrastructure for lightweight reader-writer locking
+ *
+ * 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) 2015, Red Hat, Inc.
+ *
+ * Author: Oleg Nesterov <oleg@redhat.com>
+ */
+
+#include <linux/rcu_sync.h>
+#include <linux/sched.h>
+
+enum { GP_IDLE = 0, GP_PENDING, GP_PASSED };
+enum { CB_IDLE = 0, CB_PENDING, CB_REPLAY };
+
+#define	rss_lock	gp_wait.lock
+
+/**
+ * rcu_sync_init() - Initialize an rcu_sync structure
+ * @rsp: Pointer to rcu_sync structure to be initialized
+ * @type: Flavor of RCU with which to synchronize rcu_sync structure
+ */
+void rcu_sync_init(struct rcu_sync *rsp, enum rcu_sync_type type)
+{
+	memset(rsp, 0, sizeof(*rsp));
+	init_waitqueue_head(&rsp->gp_wait);
+
+	switch (type) {
+	case RCU_SYNC:
+		rsp->sync = synchronize_rcu;
+		rsp->call = call_rcu;
+		break;
+
+	case RCU_SCHED_SYNC:
+		rsp->sync = synchronize_sched;
+		rsp->call = call_rcu_sched;
+		break;
+
+	case RCU_BH_SYNC:
+		rsp->sync = synchronize_rcu_bh;
+		rsp->call = call_rcu_bh;
+		break;
+	}
+}
+
+/**
+ * rcu_sync_enter() - Force readers onto slowpath
+ * @rsp: Pointer to rcu_sync structure to use for synchronization
+ *
+ * This function is used by updaters who need readers to make use of
+ * a slowpath during the update.  After this function returns, all
+ * subsequent calls to rcu_sync_is_idle() will return false, which
+ * tells readers to stay off their fastpaths.  A later call to
+ * rcu_sync_exit() re-enables reader slowpaths.
+ *
+ * When called in isolation, rcu_sync_enter() must wait for a grace
+ * period, however, closely spaced calls to rcu_sync_enter() can
+ * optimize away the grace-period wait via a state machine implemented
+ * by rcu_sync_enter(), rcu_sync_exit(), and rcu_sync_func().
+ */
+void rcu_sync_enter(struct rcu_sync *rsp)
+{
+	bool need_wait, need_sync;
+
+	spin_lock_irq(&rsp->rss_lock);
+	need_wait = rsp->gp_count++;
+	need_sync = rsp->gp_state == GP_IDLE;
+	if (need_sync)
+		rsp->gp_state = GP_PENDING;
+	spin_unlock_irq(&rsp->rss_lock);
+
+	BUG_ON(need_wait && need_sync);
+
+	if (need_sync) {
+		rsp->sync();
+		rsp->gp_state = GP_PASSED;
+		wake_up_all(&rsp->gp_wait);
+	} else if (need_wait) {
+		wait_event(rsp->gp_wait, rsp->gp_state == GP_PASSED);
+	} else {
+		/*
+		 * Possible when there's a pending CB from a rcu_sync_exit().
+		 * Nobody has yet been allowed the 'fast' path and thus we can
+		 * avoid doing any sync(). The callback will get 'dropped'.
+		 */
+		BUG_ON(rsp->gp_state != GP_PASSED);
+	}
+}
+
+/**
+ * rcu_sync_func() - Callback function managing reader access to fastpath
+ * @rsp: Pointer to rcu_sync structure to use for synchronization
+ *
+ * This function is passed to one of the call_rcu() functions by
+ * rcu_sync_exit(), so that it is invoked after a grace period following the
+ * that invocation of rcu_sync_exit().  It takes action based on events that
+ * have taken place in the meantime, so that closely spaced rcu_sync_enter()
+ * and rcu_sync_exit() pairs need not wait for a grace period.
+ *
+ * If another rcu_sync_enter() is invoked before the grace period
+ * ended, reset state to allow the next rcu_sync_exit() to let the
+ * readers back onto their fastpaths (after a grace period).  If both
+ * another rcu_sync_enter() and its matching rcu_sync_exit() are invoked
+ * before the grace period ended, re-invoke call_rcu() on behalf of that
+ * rcu_sync_exit().  Otherwise, set all state back to idle so that readers
+ * can again use their fastpaths.
+ */
+static void rcu_sync_func(struct rcu_head *rcu)
+{
+	struct rcu_sync *rsp = container_of(rcu, struct rcu_sync, cb_head);
+	unsigned long flags;
+
+	BUG_ON(rsp->gp_state != GP_PASSED);
+	BUG_ON(rsp->cb_state == CB_IDLE);
+
+	spin_lock_irqsave(&rsp->rss_lock, flags);
+	if (rsp->gp_count) {
+		/*
+		 * A new rcu_sync_begin() has happened; drop the callback.
+		 */
+		rsp->cb_state = CB_IDLE;
+	} else if (rsp->cb_state == CB_REPLAY) {
+		/*
+		 * A new rcu_sync_exit() has happened; requeue the callback
+		 * to catch a later GP.
+		 */
+		rsp->cb_state = CB_PENDING;
+		rsp->call(&rsp->cb_head, rcu_sync_func);
+	} else {
+		/*
+		 * We're at least a GP after rcu_sync_exit(); eveybody will now
+		 * have observed the write side critical section. Let 'em rip!.
+		 */
+		rsp->cb_state = CB_IDLE;
+		rsp->gp_state = GP_IDLE;
+	}
+	spin_unlock_irqrestore(&rsp->rss_lock, flags);
+}
+
+/**
+ * rcu_sync_exit() - Allow readers back onto fast patch after grace period
+ * @rsp: Pointer to rcu_sync structure to use for synchronization
+ *
+ * This function is used by updaters who have completed, and can therefore
+ * now allow readers to make use of their fastpaths after a grace period
+ * has elapsed.  After this grace period has completed, all subsequent
+ * calls to rcu_sync_is_idle() will return true, which tells readers that
+ * they can once again use their fastpaths.
+ */
+void rcu_sync_exit(struct rcu_sync *rsp)
+{
+	spin_lock_irq(&rsp->rss_lock);
+	if (!--rsp->gp_count) {
+		if (rsp->cb_state == CB_IDLE) {
+			rsp->cb_state = CB_PENDING;
+			rsp->call(&rsp->cb_head, rcu_sync_func);
+		} else if (rsp->cb_state == CB_PENDING) {
+			rsp->cb_state = CB_REPLAY;
+		}
+	}
+	spin_unlock_irq(&rsp->rss_lock);
+}
-- 
1.8.1.5

[PATCH RFC tip/core/rcu 2/9] rcu_sync: Simplify rcu_sync using new rcu_sync_ops structure

[Paul E. McKenney]

From: Oleg Nesterov <oleg@redhat.com>

This commit adds the new struct rcu_sync_ops which holds sync/call
methods, and turns the function pointers in rcu_sync_struct into an array
of struct rcu_sync_ops.  This simplifies the "init" helpers by collapsing
a switch statement and explicit multiple definitions into a simple
assignment and a helper macro, respectively.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 include/linux/rcu_sync.h | 60 +++++++++++++++++++-----------------------------
 kernel/rcu/sync.c        | 42 +++++++++++++++++----------------
 2 files changed, 45 insertions(+), 57 deletions(-)

diff --git a/include/linux/rcu_sync.h b/include/linux/rcu_sync.h
index cb044df2e21c..c6d2272c4459 100644
--- a/include/linux/rcu_sync.h
+++ b/include/linux/rcu_sync.h
@@ -26,6 +26,8 @@
 #include <linux/wait.h>
 #include <linux/rcupdate.h>
 
+enum rcu_sync_type { RCU_SYNC, RCU_SCHED_SYNC, RCU_BH_SYNC };
+
 /* Structure to mediate between updaters and fastpath-using readers.  */
 struct rcu_sync {
 	int			gp_state;
@@ -35,43 +37,9 @@ struct rcu_sync {
 	int			cb_state;
 	struct rcu_head		cb_head;
 
-	void (*sync)(void);
-	void (*call)(struct rcu_head *, void (*)(struct rcu_head *));
+	enum rcu_sync_type	gp_type;
 };
 
-#define ___RCU_SYNC_INIT(name)						\
-	.gp_state = 0,							\
-	.gp_count = 0,							\
-	.gp_wait = __WAIT_QUEUE_HEAD_INITIALIZER(name.gp_wait),		\
-	.cb_state = 0
-
-#define __RCU_SCHED_SYNC_INIT(name) {					\
-	___RCU_SYNC_INIT(name),						\
-	.sync = synchronize_sched,					\
-	.call = call_rcu_sched,						\
-}
-
-#define __RCU_BH_SYNC_INIT(name) {					\
-	___RCU_SYNC_INIT(name),						\
-	.sync = synchronize_rcu_bh,					\
-	.call = call_rcu_bh,						\
-}
-
-#define __RCU_SYNC_INIT(name) {						\
-	___RCU_SYNC_INIT(name),						\
-	.sync = synchronize_rcu,					\
-	.call = call_rcu,						\
-}
-
-#define DEFINE_RCU_SCHED_SYNC(name)					\
-	struct rcu_sync name = __RCU_SCHED_SYNC_INIT(name)
-
-#define DEFINE_RCU_BH_SYNC(name)					\
-	struct rcu_sync name = __RCU_BH_SYNC_INIT(name)
-
-#define DEFINE_RCU_SYNC(name)						\
-	struct rcu_sync name = __RCU_SYNC_INIT(name)
-
 /**
  * rcu_sync_is_idle() - Are readers permitted to use their fastpaths?
  * @rsp: Pointer to rcu_sync structure to use for synchronization
@@ -85,10 +53,28 @@ static inline bool rcu_sync_is_idle(struct rcu_sync *rsp)
 	return !rsp->gp_state; /* GP_IDLE */
 }
 
-enum rcu_sync_type { RCU_SYNC, RCU_SCHED_SYNC, RCU_BH_SYNC };
-
 extern void rcu_sync_init(struct rcu_sync *, enum rcu_sync_type);
 extern void rcu_sync_enter(struct rcu_sync *);
 extern void rcu_sync_exit(struct rcu_sync *);
 
+#define __RCU_SYNC_INITIALIZER(name, type) {				\
+		.gp_state = 0,						\
+		.gp_count = 0,						\
+		.gp_wait = __WAIT_QUEUE_HEAD_INITIALIZER(name.gp_wait),	\
+		.cb_state = 0,						\
+		.gp_type = type,					\
+	}
+
+#define	__DEFINE_RCU_SYNC(name, type)	\
+	struct rcu_sync_struct name = __RCU_SYNC_INITIALIZER(name, type)
+
+#define DEFINE_RCU_SYNC(name)		\
+	__DEFINE_RCU_SYNC(name, RCU_SYNC)
+
+#define DEFINE_RCU_SCHED_SYNC(name)	\
+	__DEFINE_RCU_SYNC(name, RCU_SCHED_SYNC)
+
+#define DEFINE_RCU_BH_SYNC(name)	\
+	__DEFINE_RCU_SYNC(name, RCU_BH_SYNC)
+
 #endif /* _LINUX_RCU_SYNC_H_ */
diff --git a/kernel/rcu/sync.c b/kernel/rcu/sync.c
index 0a11df43be23..5a9aa4c394f1 100644
--- a/kernel/rcu/sync.c
+++ b/kernel/rcu/sync.c
@@ -23,6 +23,24 @@
 #include <linux/rcu_sync.h>
 #include <linux/sched.h>
 
+static const struct {
+	void (*sync)(void);
+	void (*call)(struct rcu_head *, void (*)(struct rcu_head *));
+} gp_ops[] = {
+	[RCU_SYNC] = {
+		.sync = synchronize_rcu,
+		.call = call_rcu,
+	},
+	[RCU_SCHED_SYNC] = {
+		.sync = synchronize_sched,
+		.call = call_rcu_sched,
+	},
+	[RCU_BH_SYNC] = {
+		.sync = synchronize_rcu_bh,
+		.call = call_rcu_bh,
+	},
+};
+
 enum { GP_IDLE = 0, GP_PENDING, GP_PASSED };
 enum { CB_IDLE = 0, CB_PENDING, CB_REPLAY };
 
@@ -37,23 +55,7 @@ void rcu_sync_init(struct rcu_sync *rsp, enum rcu_sync_type type)
 {
 	memset(rsp, 0, sizeof(*rsp));
 	init_waitqueue_head(&rsp->gp_wait);
-
-	switch (type) {
-	case RCU_SYNC:
-		rsp->sync = synchronize_rcu;
-		rsp->call = call_rcu;
-		break;
-
-	case RCU_SCHED_SYNC:
-		rsp->sync = synchronize_sched;
-		rsp->call = call_rcu_sched;
-		break;
-
-	case RCU_BH_SYNC:
-		rsp->sync = synchronize_rcu_bh;
-		rsp->call = call_rcu_bh;
-		break;
-	}
+	rsp->gp_type = type;
 }
 
 /**
@@ -85,7 +87,7 @@ void rcu_sync_enter(struct rcu_sync *rsp)
 	BUG_ON(need_wait && need_sync);
 
 	if (need_sync) {
-		rsp->sync();
+		gp_ops[rsp->gp_type].sync();
 		rsp->gp_state = GP_PASSED;
 		wake_up_all(&rsp->gp_wait);
 	} else if (need_wait) {
@@ -138,7 +140,7 @@ static void rcu_sync_func(struct rcu_head *rcu)
 		 * to catch a later GP.
 		 */
 		rsp->cb_state = CB_PENDING;
-		rsp->call(&rsp->cb_head, rcu_sync_func);
+		gp_ops[rsp->gp_type].call(&rsp->cb_head, rcu_sync_func);
 	} else {
 		/*
 		 * We're at least a GP after rcu_sync_exit(); eveybody will now
@@ -166,7 +168,7 @@ void rcu_sync_exit(struct rcu_sync *rsp)
 	if (!--rsp->gp_count) {
 		if (rsp->cb_state == CB_IDLE) {
 			rsp->cb_state = CB_PENDING;
-			rsp->call(&rsp->cb_head, rcu_sync_func);
+			gp_ops[rsp->gp_type].call(&rsp->cb_head, rcu_sync_func);
 		} else if (rsp->cb_state == CB_PENDING) {
 			rsp->cb_state = CB_REPLAY;
 		}
-- 
1.8.1.5

[PATCH RFC tip/core/rcu 3/9] rcu_sync: Add CONFIG_PROVE_RCU checks

[Paul E. McKenney]

From: Oleg Nesterov <oleg@redhat.com>

This commit validates that the caller of rcu_sync_is_idle() holds the
corresponding type of RCU read-side lock, but only in kernels built
with CONFIG_PROVE_RCU=y.  This validation is carried out via a new
rcu_sync_ops->held() method that is checked within rcu_sync_is_idle().

Note that although this does add code to the fast path, it only does so
in kernels built with CONFIG_PROVE_RCU=y.

Suggested-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 include/linux/rcu_sync.h |  6 ++++++
 kernel/rcu/sync.c        | 20 ++++++++++++++++++++
 2 files changed, 26 insertions(+)

diff --git a/include/linux/rcu_sync.h b/include/linux/rcu_sync.h
index c6d2272c4459..c55a070b2592 100644
--- a/include/linux/rcu_sync.h
+++ b/include/linux/rcu_sync.h
@@ -40,6 +40,8 @@ struct rcu_sync {
 	enum rcu_sync_type	gp_type;
 };
 
+extern bool __rcu_sync_is_idle(struct rcu_sync *);
+
 /**
  * rcu_sync_is_idle() - Are readers permitted to use their fastpaths?
  * @rsp: Pointer to rcu_sync structure to use for synchronization
@@ -50,7 +52,11 @@ struct rcu_sync {
  */
 static inline bool rcu_sync_is_idle(struct rcu_sync *rsp)
 {
+#ifdef CONFIG_PROVE_RCU
+	return __rcu_sync_is_idle(rss);
+#else
 	return !rsp->gp_state; /* GP_IDLE */
+#endif
 }
 
 extern void rcu_sync_init(struct rcu_sync *, enum rcu_sync_type);
diff --git a/kernel/rcu/sync.c b/kernel/rcu/sync.c
index 5a9aa4c394f1..26b2629e731e 100644
--- a/kernel/rcu/sync.c
+++ b/kernel/rcu/sync.c
@@ -23,21 +23,33 @@
 #include <linux/rcu_sync.h>
 #include <linux/sched.h>
 
+#ifdef CONFIG_PROVE_RCU
+#define __INIT_HELD(func)	.held = func,
+#else
+#define __INIT_HELD(func)
+#endif
+
 static const struct {
 	void (*sync)(void);
 	void (*call)(struct rcu_head *, void (*)(struct rcu_head *));
+#ifdef CONFIG_PROVE_RCU
+	int  (*held)(void);
+#endif
 } gp_ops[] = {
 	[RCU_SYNC] = {
 		.sync = synchronize_rcu,
 		.call = call_rcu,
+		__INIT_HELD(rcu_read_lock_held)
 	},
 	[RCU_SCHED_SYNC] = {
 		.sync = synchronize_sched,
 		.call = call_rcu_sched,
+		__INIT_HELD(rcu_read_lock_sched_held)
 	},
 	[RCU_BH_SYNC] = {
 		.sync = synchronize_rcu_bh,
 		.call = call_rcu_bh,
+		__INIT_HELD(rcu_read_lock_bh_held)
 	},
 };
 
@@ -46,6 +58,13 @@ enum { CB_IDLE = 0, CB_PENDING, CB_REPLAY };
 
 #define	rss_lock	gp_wait.lock
 
+#ifdef CONFIG_PROVE_RCU
+bool __rcu_sync_is_idle(struct rcu_sync *rsp)
+{
+	WARN_ON(!gp_ops[rsp->gp_type].held());
+	return rsp->gp_state == GP_IDLE;
+}
+
 /**
  * rcu_sync_init() - Initialize an rcu_sync structure
  * @rsp: Pointer to rcu_sync structure to be initialized
@@ -57,6 +76,7 @@ void rcu_sync_init(struct rcu_sync *rsp, enum rcu_sync_type type)
 	init_waitqueue_head(&rsp->gp_wait);
 	rsp->gp_type = type;
 }
+#endif
 
 /**
  * rcu_sync_enter() - Force readers onto slowpath
-- 
1.8.1.5

[PATCH RFC tip/core/rcu 4/9] rcu_sync: Introduce rcu_sync_dtor()

[Paul E. McKenney]

From: Oleg Nesterov <oleg@redhat.com>

This commit allows rcu_sync structures to be safely deallocated,
The trick is to add a new ->wait field to the gp_ops array.
This field is a pointer to the rcu_barrier() function corresponding
to the flavor of RCU in question.  This allows a new rcu_sync_dtor()
to wait for any outstanding callbacks before freeing the rcu_sync
structure.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 include/linux/rcu_sync.h |  1 +
 kernel/rcu/sync.c        | 22 ++++++++++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/include/linux/rcu_sync.h b/include/linux/rcu_sync.h
index c55a070b2592..67a31ada392f 100644
--- a/include/linux/rcu_sync.h
+++ b/include/linux/rcu_sync.h
@@ -62,6 +62,7 @@ static inline bool rcu_sync_is_idle(struct rcu_sync *rsp)
 extern void rcu_sync_init(struct rcu_sync *, enum rcu_sync_type);
 extern void rcu_sync_enter(struct rcu_sync *);
 extern void rcu_sync_exit(struct rcu_sync *);
+extern void rcu_sync_dtor(struct rcu_sync *);
 
 #define __RCU_SYNC_INITIALIZER(name, type) {				\
 		.gp_state = 0,						\
diff --git a/kernel/rcu/sync.c b/kernel/rcu/sync.c
index 26b2629e731e..a1f87f1bb705 100644
--- a/kernel/rcu/sync.c
+++ b/kernel/rcu/sync.c
@@ -32,6 +32,7 @@
 static const struct {
 	void (*sync)(void);
 	void (*call)(struct rcu_head *, void (*)(struct rcu_head *));
+	void (*wait)(void);
 #ifdef CONFIG_PROVE_RCU
 	int  (*held)(void);
 #endif
@@ -39,16 +40,19 @@ static const struct {
 	[RCU_SYNC] = {
 		.sync = synchronize_rcu,
 		.call = call_rcu,
+		.wait = rcu_barrier,
 		__INIT_HELD(rcu_read_lock_held)
 	},
 	[RCU_SCHED_SYNC] = {
 		.sync = synchronize_sched,
 		.call = call_rcu_sched,
+		.wait = rcu_barrier_sched,
 		__INIT_HELD(rcu_read_lock_sched_held)
 	},
 	[RCU_BH_SYNC] = {
 		.sync = synchronize_rcu_bh,
 		.call = call_rcu_bh,
+		.wait = rcu_barrier_bh,
 		__INIT_HELD(rcu_read_lock_bh_held)
 	},
 };
@@ -195,3 +199,21 @@ void rcu_sync_exit(struct rcu_sync *rsp)
 	}
 	spin_unlock_irq(&rsp->rss_lock);
 }
+
+void rcu_sync_dtor(struct rcu_sync *rsp)
+{
+	int cb_state;
+
+	BUG_ON(rsp->gp_count);
+
+	spin_lock_irq(&rsp->rss_lock);
+	if (rsp->cb_state == CB_REPLAY)
+		rsp->cb_state = CB_PENDING;
+	cb_state = rsp->cb_state;
+	spin_unlock_irq(&rsp->rss_lock);
+
+	if (cb_state != CB_IDLE) {
+		gp_ops[rsp->gp_type].wait();
+		BUG_ON(rsp->cb_state != CB_IDLE);
+	}
+}
-- 
1.8.1.5

[PATCH RFC tip/core/rcu 5/9] locking/percpu-rwsem: Make percpu_free_rwsem() after kzalloc() safe

[Paul E. McKenney]

From: Oleg Nesterov <oleg@redhat.com>

This is the temporary ugly hack which will be reverted later. We only
need it to ensure that the next patch will not break "change sb_writers
to use percpu_rw_semaphore" patches routed via the VFS tree.

The alloc_super()->destroy_super() error path assumes that it is safe
to call percpu_free_rwsem() after kzalloc() without percpu_init_rwsem(),
so let's not disappoint it.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/locking/percpu-rwsem.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
index 652a8ee8efe9..67a758df1d7c 100644
--- a/kernel/locking/percpu-rwsem.c
+++ b/kernel/locking/percpu-rwsem.c
@@ -25,6 +25,13 @@ int __percpu_init_rwsem(struct percpu_rw_semaphore *brw,
 
 void percpu_free_rwsem(struct percpu_rw_semaphore *brw)
 {
+	/*
+	 * XXX: temporary kludge. The error path in alloc_super()
+	 * assumes that percpu_free_rwsem() is safe after kzalloc().
+	 */
+	if (!brw->fast_read_ctr)
+		return;
+
 	free_percpu(brw->fast_read_ctr);
 	brw->fast_read_ctr = NULL; /* catch use after free bugs */
 }
-- 
1.8.1.5

[PATCH RFC tip/core/rcu 6/9] locking/percpu-rwsem: Make use of the rcu_sync infrastructure

[Paul E. McKenney]

From: Oleg Nesterov <oleg@redhat.com>

Currently down_write/up_write calls synchronize_sched_expedited()
twice, which is evil.  Change this code to rely on rcu-sync primitives.
This avoids the _expedited "big hammer", and this can be faster in
the contended case or even in the case when a single thread does
down_write/up_write in a loop.

Of course, a single down_write() will take more time, but otoh it
will be much more friendly to the whole system.

To simplify the review this patch doesn't update the comments, fixed
by the next change.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 include/linux/percpu-rwsem.h  |  3 ++-
 kernel/locking/percpu-rwsem.c | 18 +++++++-----------
 2 files changed, 9 insertions(+), 12 deletions(-)

diff --git a/include/linux/percpu-rwsem.h b/include/linux/percpu-rwsem.h
index 3e88c9a7d57f..1ab2cf130816 100644
--- a/include/linux/percpu-rwsem.h
+++ b/include/linux/percpu-rwsem.h
@@ -5,11 +5,12 @@
 #include <linux/rwsem.h>
 #include <linux/percpu.h>
 #include <linux/wait.h>
+#include <linux/rcu_sync.h>
 #include <linux/lockdep.h>
 
 struct percpu_rw_semaphore {
+	struct rcu_sync		rss;
 	unsigned int __percpu	*fast_read_ctr;
-	atomic_t		write_ctr;
 	struct rw_semaphore	rw_sem;
 	atomic_t		slow_read_ctr;
 	wait_queue_head_t	write_waitq;
diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
index 67a758df1d7c..7abc0e150a22 100644
--- a/kernel/locking/percpu-rwsem.c
+++ b/kernel/locking/percpu-rwsem.c
@@ -17,7 +17,7 @@ int __percpu_init_rwsem(struct percpu_rw_semaphore *brw,
 
 	/* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */
 	__init_rwsem(&brw->rw_sem, name, rwsem_key);
-	atomic_set(&brw->write_ctr, 0);
+	rcu_sync_init(&brw->rss, RCU_SCHED_SYNC);
 	atomic_set(&brw->slow_read_ctr, 0);
 	init_waitqueue_head(&brw->write_waitq);
 	return 0;
@@ -32,6 +32,7 @@ void percpu_free_rwsem(struct percpu_rw_semaphore *brw)
 	if (!brw->fast_read_ctr)
 		return;
 
+	rcu_sync_dtor(&brw->rss);
 	free_percpu(brw->fast_read_ctr);
 	brw->fast_read_ctr = NULL; /* catch use after free bugs */
 }
@@ -61,13 +62,12 @@ void percpu_free_rwsem(struct percpu_rw_semaphore *brw)
  */
 static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val)
 {
-	bool success = false;
+	bool success;
 
 	preempt_disable();
-	if (likely(!atomic_read(&brw->write_ctr))) {
+	success = rcu_sync_is_idle(&brw->rss);
+	if (likely(success))
 		__this_cpu_add(*brw->fast_read_ctr, val);
-		success = true;
-	}
 	preempt_enable();
 
 	return success;
@@ -133,8 +133,6 @@ static int clear_fast_ctr(struct percpu_rw_semaphore *brw)
  */
 void percpu_down_write(struct percpu_rw_semaphore *brw)
 {
-	/* tell update_fast_ctr() there is a pending writer */
-	atomic_inc(&brw->write_ctr);
 	/*
 	 * 1. Ensures that write_ctr != 0 is visible to any down_read/up_read
 	 *    so that update_fast_ctr() can't succeed.
@@ -146,7 +144,7 @@ void percpu_down_write(struct percpu_rw_semaphore *brw)
 	 *    fast-path, it executes a full memory barrier before we return.
 	 *    See R_W case in the comment above update_fast_ctr().
 	 */
-	synchronize_sched_expedited();
+	rcu_sync_enter(&brw->rss);
 
 	/* exclude other writers, and block the new readers completely */
 	down_write(&brw->rw_sem);
@@ -166,7 +164,5 @@ void percpu_up_write(struct percpu_rw_semaphore *brw)
 	 * Insert the barrier before the next fast-path in down_read,
 	 * see W_R case in the comment above update_fast_ctr().
 	 */
-	synchronize_sched_expedited();
-	/* the last writer unblocks update_fast_ctr() */
-	atomic_dec(&brw->write_ctr);
+	rcu_sync_exit(&brw->rss);
 }
-- 
1.8.1.5

[PATCH RFC tip/core/rcu 7/9] locking/percpu-rwsem: Fix the comments outdated by rcu_sync

[Paul E. McKenney]

From: Oleg Nesterov <oleg@redhat.com>

Update the comments broken by the previous change.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/locking/percpu-rwsem.c | 50 ++++++++++---------------------------------
 1 file changed, 11 insertions(+), 39 deletions(-)

diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
index 7abc0e150a22..25b73448929c 100644
--- a/kernel/locking/percpu-rwsem.c
+++ b/kernel/locking/percpu-rwsem.c
@@ -38,27 +38,12 @@ void percpu_free_rwsem(struct percpu_rw_semaphore *brw)
 }
 
 /*
- * This is the fast-path for down_read/up_read, it only needs to ensure
- * there is no pending writer (atomic_read(write_ctr) == 0) and inc/dec the
- * fast per-cpu counter. The writer uses synchronize_sched_expedited() to
- * serialize with the preempt-disabled section below.
- *
- * The nontrivial part is that we should guarantee acquire/release semantics
- * in case when
- *
- *	R_W: down_write() comes after up_read(), the writer should see all
- *	     changes done by the reader
- * or
- *	W_R: down_read() comes after up_write(), the reader should see all
- *	     changes done by the writer
+ * This is the fast-path for down_read/up_read. If it succeeds we rely
+ * on the barriers provided by rcu_sync_enter/exit; see the comments in
+ * percpu_down_write() and percpu_up_write().
  *
  * If this helper fails the callers rely on the normal rw_semaphore and
  * atomic_dec_and_test(), so in this case we have the necessary barriers.
- *
- * But if it succeeds we do not have any barriers, atomic_read(write_ctr) or
- * __this_cpu_add() below can be reordered with any LOAD/STORE done by the
- * reader inside the critical section. See the comments in down_write and
- * up_write below.
  */
 static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val)
 {
@@ -120,29 +105,15 @@ static int clear_fast_ctr(struct percpu_rw_semaphore *brw)
 	return sum;
 }
 
-/*
- * A writer increments ->write_ctr to force the readers to switch to the
- * slow mode, note the atomic_read() check in update_fast_ctr().
- *
- * After that the readers can only inc/dec the slow ->slow_read_ctr counter,
- * ->fast_read_ctr is stable. Once the writer moves its sum into the slow
- * counter it represents the number of active readers.
- *
- * Finally the writer takes ->rw_sem for writing and blocks the new readers,
- * then waits until the slow counter becomes zero.
- */
 void percpu_down_write(struct percpu_rw_semaphore *brw)
 {
 	/*
-	 * 1. Ensures that write_ctr != 0 is visible to any down_read/up_read
-	 *    so that update_fast_ctr() can't succeed.
-	 *
-	 * 2. Ensures we see the result of every previous this_cpu_add() in
-	 *    update_fast_ctr().
+	 * Make rcu_sync_is_idle() == F and thus disable the fast-path in
+	 * percpu_down_read() and percpu_up_read(), and wait for gp pass.
 	 *
-	 * 3. Ensures that if any reader has exited its critical section via
-	 *    fast-path, it executes a full memory barrier before we return.
-	 *    See R_W case in the comment above update_fast_ctr().
+	 * The latter synchronises us with the preceding readers which used
+	 * the fast-past, so we can not miss the result of __this_cpu_add()
+	 * or anything else inside their criticial sections.
 	 */
 	rcu_sync_enter(&brw->rss);
 
@@ -161,8 +132,9 @@ void percpu_up_write(struct percpu_rw_semaphore *brw)
 	/* release the lock, but the readers can't use the fast-path */
 	up_write(&brw->rw_sem);
 	/*
-	 * Insert the barrier before the next fast-path in down_read,
-	 * see W_R case in the comment above update_fast_ctr().
+	 * Enable the fast-path in percpu_down_read() and percpu_up_read()
+	 * but only after another gp pass; this adds the necessary barrier
+	 * to ensure the reader can't miss the changes done by us.
 	 */
 	rcu_sync_exit(&brw->rss);
 }
-- 
1.8.1.5

[PATCH RFC tip/core/rcu 8/9] locking/percpu-rwsem: Clean up the lockdep annotations in percpu_down_read()

[Paul E. McKenney]

From: Oleg Nesterov <oleg@redhat.com>

Based on Peter Zijlstra's earlier patch.

Change percpu_down_read() to use __down_read(), this way we can
do rwsem_acquire_read() unconditionally at the start to make this
code more symmetric and clean.

Originally-From: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/locking/percpu-rwsem.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
index 25b73448929c..61b678d784ce 100644
--- a/kernel/locking/percpu-rwsem.c
+++ b/kernel/locking/percpu-rwsem.c
@@ -69,14 +69,14 @@ static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val)
 void percpu_down_read(struct percpu_rw_semaphore *brw)
 {
 	might_sleep();
-	if (likely(update_fast_ctr(brw, +1))) {
-		rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 0, _RET_IP_);
+	rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 0, _RET_IP_);
+
+	if (likely(update_fast_ctr(brw, +1)))
 		return;
-	}
 
-	down_read(&brw->rw_sem);
+	/* Avoid rwsem_acquire_read() and rwsem_release() */
+	__down_read(&brw->rw_sem);
 	atomic_inc(&brw->slow_read_ctr);
-	/* avoid up_read()->rwsem_release() */
 	__up_read(&brw->rw_sem);
 }
 
-- 
1.8.1.5

[PATCH RFC tip/core/rcu 9/9] rcu: Change _wait_rcu_gp() to work around GCC bug 67055

[Paul E. McKenney]

From: Oleg Nesterov <oleg@redhat.com>

Code like this in inline functions confuses some recent versions of gcc:

	const int n = const-expr;
	whatever_t array[n];

For more details, see:

	https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67055#c13

This compiler bug results in the following failure after 114b7fd4b (rcu:
Create rcu_sync infrastructure):

	In file included from include/linux/rcupdate.h:429:0,
			  from include/linux/rcu_sync.h:5,
			  from kernel/rcu/sync.c:1:
	include/linux/rcutiny.h: In function 'rcu_barrier_sched':
	include/linux/rcutiny.h:55:20: internal compiler error: Segmentation fault
	  static inline void rcu_barrier_sched(void)

This commit therefore eliminates the constant local variable in favor of
direct use of the expression.

Reported-and-tested-by: Mark Salter <msalter@redhat.com>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 include/linux/rcupdate.h | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index ff476515f716..581abf848566 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -230,12 +230,11 @@ void __wait_rcu_gp(bool checktiny, int n, call_rcu_func_t *crcu_array,
 		   struct rcu_synchronize *rs_array);
 
 #define _wait_rcu_gp(checktiny, ...) \
-do { \
-	call_rcu_func_t __crcu_array[] = { __VA_ARGS__ }; \
-	const int __n = ARRAY_SIZE(__crcu_array); \
-	struct rcu_synchronize __rs_array[__n]; \
-	\
-	__wait_rcu_gp(checktiny, __n, __crcu_array, __rs_array); \
+do {									\
+	call_rcu_func_t __crcu_array[] = { __VA_ARGS__ };		\
+	struct rcu_synchronize __rs_array[ARRAY_SIZE(__crcu_array)];	\
+	__wait_rcu_gp(checktiny, ARRAY_SIZE(__crcu_array),		\
+			__crcu_array, __rs_array);			\
 } while (0)
 
 #define wait_rcu_gp(...) _wait_rcu_gp(false, __VA_ARGS__)
-- 
1.8.1.5

Re: [PATCH RFC tip/core/rcu 3/9] rcu_sync: Add CONFIG_PROVE_RCU checks

[Daniel Wagner]

On 08/29/2015 05:35 AM, Paul E. McKenney wrote:
> +extern bool __rcu_sync_is_idle(struct rcu_sync *);
> +
>  /**
>   * rcu_sync_is_idle() - Are readers permitted to use their fastpaths?
>   * @rsp: Pointer to rcu_sync structure to use for synchronization
> @@ -50,7 +52,11 @@ struct rcu_sync {
>   */
>  static inline bool rcu_sync_is_idle(struct rcu_sync *rsp)
>  {
> +#ifdef CONFIG_PROVE_RCU
> +	return __rcu_sync_is_idle(rss);

s/rss/rsp?

> +#else
>  	return !rsp->gp_state; /* GP_IDLE */
> +#endif
>  }

Re: [PATCH RFC tip/core/rcu 3/9] rcu_sync: Add CONFIG_PROVE_RCU checks

[Oleg Nesterov]

On 09/07, Daniel Wagner wrote:
>
> On 08/29/2015 05:35 AM, Paul E. McKenney wrote:
> > +extern bool __rcu_sync_is_idle(struct rcu_sync *);
> > +
> >  /**
> >   * rcu_sync_is_idle() - Are readers permitted to use their fastpaths?
> >   * @rsp: Pointer to rcu_sync structure to use for synchronization
> > @@ -50,7 +52,11 @@ struct rcu_sync {
> >   */
> >  static inline bool rcu_sync_is_idle(struct rcu_sync *rsp)
> >  {
> > +#ifdef CONFIG_PROVE_RCU
> > +	return __rcu_sync_is_idle(rss);
>
> s/rss/rsp?

Hmm, yes.

Paul, it seems that you renamed "rss" to "rsp" globally (I didn't even
notice this change), but forgot about this one ;)

Should I send the patch or will you fix this yourself?

Oleg.

Re: [PATCH RFC tip/core/rcu 3/9] rcu_sync: Add CONFIG_PROVE_RCU checks

[Paul E. McKenney]

On Mon, Sep 07, 2015 at 02:52:47PM +0200, Oleg Nesterov wrote:
> On 09/07, Daniel Wagner wrote:
> >
> > On 08/29/2015 05:35 AM, Paul E. McKenney wrote:
> > > +extern bool __rcu_sync_is_idle(struct rcu_sync *);
> > > +
> > >  /**
> > >   * rcu_sync_is_idle() - Are readers permitted to use their fastpaths?
> > >   * @rsp: Pointer to rcu_sync structure to use for synchronization
> > > @@ -50,7 +52,11 @@ struct rcu_sync {
> > >   */
> > >  static inline bool rcu_sync_is_idle(struct rcu_sync *rsp)
> > >  {
> > > +#ifdef CONFIG_PROVE_RCU
> > > +	return __rcu_sync_is_idle(rss);
> >
> > s/rss/rsp?
> 
> Hmm, yes.
> 
> Paul, it seems that you renamed "rss" to "rsp" globally (I didn't even
> notice this change), but forgot about this one ;)
> 
> Should I send the patch or will you fix this yourself?

I did fix it in -rcu, and a few other breakages as well.  Most of them
self-inflicted, as usual.  However, I am holding off on this series in
order to get it a bit more pre-merge testing.  I expect to push this
in v4.4.

							Thanx, Paul