[PATCH tip/core/rcu 0/4] SRCU updates

[PATCH tip/core/rcu 0/4] SRCU updates

[Paul E. McKenney]

Hello!

This series contains a few SRCU updates:

1.	Move rcu_scheduler_starting() from Tiny RCU to Tiny SRCU.

2.	Remove obsolete SRCU-C.boot kernel-parameter file

3.	Make process_srcu() be static.

4.	Provide ordering for CPU not involved in grace period.

							Thanx, Paul

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

 b/include/linux/rcupdate.h                                 |    1 
 b/include/linux/rcutiny.h                                  |    8 --
 b/include/linux/srcutree.h                                 |    2 
 b/kernel/rcu/srcutiny.c                                    |    8 ++
 b/kernel/rcu/srcutree.c                                    |   13 +++
 b/kernel/rcu/tiny.c                                        |    2 
 kernel/rcu/tiny_plugin.h                                   |   47 -------------
 tools/testing/selftests/rcutorture/configs/rcu/SRCU-C.boot |    1 
 8 files changed, 23 insertions(+), 59 deletions(-)

[PATCH tip/core/rcu 1/4] srcu: Move rcu_scheduler_starting() from Tiny RCU to Tiny SRCU

[Paul E. McKenney]

Other than lockdep support, Tiny RCU has no need for the
scheduler status.  However, Tiny SRCU will need this to control
boot-time behavior independent of lockdep.  Therefore, this commit
moves rcu_scheduler_starting() from kernel/rcu/tiny_plugin.h to
kernel/rcu/srcutiny.c.  This in turn allows the complete removal of
kernel/rcu/tiny_plugin.h.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 include/linux/rcupdate.h |  1 +
 include/linux/rcutiny.h  |  8 +++-----
 kernel/rcu/srcutiny.c    |  8 ++++++++
 kernel/rcu/tiny.c        |  2 --
 kernel/rcu/tiny_plugin.h | 47 -----------------------------------------------
 5 files changed, 12 insertions(+), 54 deletions(-)
 delete mode 100644 kernel/rcu/tiny_plugin.h

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index f816fc72b51e..55c29e1650b9 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -105,6 +105,7 @@ static inline int rcu_preempt_depth(void)
 
 /* Internal to kernel */
 void rcu_init(void);
+extern int rcu_scheduler_active __read_mostly;
 void rcu_sched_qs(void);
 void rcu_bh_qs(void);
 void rcu_check_callbacks(int user);
diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index 5becbbccb998..b3dbf9502fd0 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -116,13 +116,11 @@ static inline void rcu_irq_exit_irqson(void) { }
 static inline void rcu_irq_enter_irqson(void) { }
 static inline void rcu_irq_exit(void) { }
 static inline void exit_rcu(void) { }
-
-#if defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_SRCU)
-extern int rcu_scheduler_active __read_mostly;
+#ifdef CONFIG_SRCU
 void rcu_scheduler_starting(void);
-#else /* #if defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_SRCU) */
+#else /* #ifndef CONFIG_SRCU */
 static inline void rcu_scheduler_starting(void) { }
-#endif /* #else #if defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_SRCU) */
+#endif /* #else #ifndef CONFIG_SRCU */
 static inline void rcu_end_inkernel_boot(void) { }
 static inline bool rcu_is_watching(void) { return true; }
 
diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
index 1a1c1047d2ed..76ac5f50b2c7 100644
--- a/kernel/rcu/srcutiny.c
+++ b/kernel/rcu/srcutiny.c
@@ -33,6 +33,8 @@
 #include "rcu_segcblist.h"
 #include "rcu.h"
 
+int rcu_scheduler_active __read_mostly;
+
 static int init_srcu_struct_fields(struct srcu_struct *sp)
 {
 	sp->srcu_lock_nesting[0] = 0;
@@ -193,3 +195,9 @@ void synchronize_srcu(struct srcu_struct *sp)
 	destroy_rcu_head_on_stack(&rs.head);
 }
 EXPORT_SYMBOL_GPL(synchronize_srcu);
+
+/* Lockdep diagnostics.  */
+void __init rcu_scheduler_starting(void)
+{
+	rcu_scheduler_active = RCU_SCHEDULER_RUNNING;
+}
diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c
index f8488965250f..a64eee0db39e 100644
--- a/kernel/rcu/tiny.c
+++ b/kernel/rcu/tiny.c
@@ -56,8 +56,6 @@ static struct rcu_ctrlblk rcu_bh_ctrlblk = {
 	.curtail	= &rcu_bh_ctrlblk.rcucblist,
 };
 
-#include "tiny_plugin.h"
-
 void rcu_barrier_bh(void)
 {
 	wait_rcu_gp(call_rcu_bh);
diff --git a/kernel/rcu/tiny_plugin.h b/kernel/rcu/tiny_plugin.h
deleted file mode 100644
index f0a01b2a3062..000000000000
--- a/kernel/rcu/tiny_plugin.h
+++ /dev/null
@@ -1,47 +0,0 @@
-/*
- * Read-Copy Update mechanism for mutual exclusion, the Bloatwatch edition
- * Internal non-public definitions that provide either classic
- * or preemptible semantics.
- *
- * 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) 2010 Linaro
- *
- * Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
- */
-
-#if defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_SRCU)
-#include <linux/kernel_stat.h>
-
-int rcu_scheduler_active __read_mostly;
-EXPORT_SYMBOL_GPL(rcu_scheduler_active);
-
-/*
- * During boot, we forgive RCU lockdep issues.  After this function is
- * invoked, we start taking RCU lockdep issues seriously.  Note that unlike
- * Tree RCU, Tiny RCU transitions directly from RCU_SCHEDULER_INACTIVE
- * to RCU_SCHEDULER_RUNNING, skipping the RCU_SCHEDULER_INIT stage.
- * The reason for this is that Tiny RCU does not need kthreads, so does
- * not have to care about the fact that the scheduler is half-initialized
- * at a certain phase of the boot process.  Unless SRCU is in the mix.
- */
-void __init rcu_scheduler_starting(void)
-{
-	WARN_ON(nr_context_switches() > 0);
-	rcu_scheduler_active = IS_ENABLED(CONFIG_SRCU)
-		? RCU_SCHEDULER_INIT : RCU_SCHEDULER_RUNNING;
-}
-
-#endif /* #if defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_SRCU) */
-- 
2.5.2

[PATCH tip/core/rcu 2/4] rcutorture: Remove obsolete SRCU-C.boot

[Paul E. McKenney]

Classic SRCU is no more, so this commit removes the corresponding
rcutorture boot-parameters file.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 tools/testing/selftests/rcutorture/configs/rcu/SRCU-C.boot | 1 -
 1 file changed, 1 deletion(-)
 delete mode 100644 tools/testing/selftests/rcutorture/configs/rcu/SRCU-C.boot

diff --git a/tools/testing/selftests/rcutorture/configs/rcu/SRCU-C.boot b/tools/testing/selftests/rcutorture/configs/rcu/SRCU-C.boot
deleted file mode 100644
index 84a7d51b7481..000000000000
--- a/tools/testing/selftests/rcutorture/configs/rcu/SRCU-C.boot
+++ /dev/null
@@ -1 +0,0 @@
-rcutorture.torture_type=srcud
-- 
2.5.2

[PATCH tip/core/rcu 3/4] srcu: Make process_srcu() be static

[Paul E. McKenney]

The function process_srcu() is not invoked outside of srcutree.c, so
this commit makes it static and drops the EXPORT_SYMBOL_GPL().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 include/linux/srcutree.h | 2 --
 kernel/rcu/srcutree.c    | 4 ++--
 2 files changed, 2 insertions(+), 4 deletions(-)

diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
index 42973f787e7e..a026a9493bde 100644
--- a/include/linux/srcutree.h
+++ b/include/linux/srcutree.h
@@ -104,8 +104,6 @@ struct srcu_struct {
 #define SRCU_STATE_SCAN1	1
 #define SRCU_STATE_SCAN2	2
 
-void process_srcu(struct work_struct *work);
-
 #define __SRCU_STRUCT_INIT(name)					\
 	{								\
 		.sda = &name##_srcu_data,				\
diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index d0ca524bf042..94bd6ed43ea3 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -51,6 +51,7 @@ module_param(counter_wrap_check, ulong, 0444);
 
 static void srcu_invoke_callbacks(struct work_struct *work);
 static void srcu_reschedule(struct srcu_struct *sp, unsigned long delay);
+static void process_srcu(struct work_struct *work);
 
 /*
  * Initialize SRCU combining tree.  Note that statically allocated
@@ -1194,7 +1195,7 @@ static void srcu_reschedule(struct srcu_struct *sp, unsigned long delay)
 /*
  * This is the work-queue function that handles SRCU grace periods.
  */
-void process_srcu(struct work_struct *work)
+static void process_srcu(struct work_struct *work)
 {
 	struct srcu_struct *sp;
 
@@ -1203,7 +1204,6 @@ void process_srcu(struct work_struct *work)
 	srcu_advance_state(sp);
 	srcu_reschedule(sp, srcu_get_delay(sp));
 }
-EXPORT_SYMBOL_GPL(process_srcu);
 
 void srcutorture_get_gp_data(enum rcutorture_type test_type,
 			     struct srcu_struct *sp, int *flags,
-- 
2.5.2

[PATCH tip/core/rcu 4/4] srcu: Provide ordering for CPU not involved in grace period

[Paul E. McKenney]

Tree RCU guarantees that every online CPU has a memory barrier between
any given grace period and any of that CPU's RCU read-side sections that
must be ordered against that grace period.  Since RCU doesn't always
know where read-side critical sections are, the actual implementation
guarantees order against prior and subsequent non-idle non-offline code,
whether in an RCU read-side critical section or not.  As a result, there
does not need to be a memory barrier at the end of synchronize_rcu()
and friends because the ordering internal to the grace period has
ordered every CPU's post-grace-period execution against each CPU's
pre-grace-period execution, again for all non-idle online CPUs.

In contrast, SRCU can have non-idle online CPUs that are completely
uninvolved in a given SRCU grace period, for example, a CPU that
never runs any SRCU read-side critical sections and took no part in
the grace-period processing.  It is in theory possible for a given
synchronize_srcu()'s wakeup to be delivered to a CPU that was completely
uninvolved in the prior SRCU grace period, which could mean that the
code following that synchronize_srcu() would end up being unordered with
respect to both the grace period and any pre-existing SRCU read-side
critical sections.

This commit therefore adds an smp_mb() to the end of __synchronize_srcu(),
which prevents this scenario from occurring.

Reported-by: Lance Roy <ldr709@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Lance Roy <ldr709@gmail.com>
---
 kernel/rcu/srcutree.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 94bd6ed43ea3..c1c0ee3cce3b 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -897,6 +897,15 @@ static void __synchronize_srcu(struct srcu_struct *sp, bool do_norm)
 	__call_srcu(sp, &rcu.head, wakeme_after_rcu, do_norm);
 	wait_for_completion(&rcu.completion);
 	destroy_rcu_head_on_stack(&rcu.head);
+
+	/*
+	 * Make sure that later code is ordered after the SRCU grace
+	 * period.  This pairs with the raw_spin_lock_irq_rcu_node()
+	 * in srcu_invoke_callbacks().  Unlike Tree RCU, this is needed
+	 * because the current CPU might have been totally uninvolved with
+	 * (and thus unordered against) that grace period.
+	 */
+	smp_mb();
 }
 
 /**
-- 
2.5.2