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Date:	Mon, 22 Sep 2014 09:25:34 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	Pranith Kumar <bobby.prani@...il.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] rcu: Remove redundant CONFIG_PREEMPT_RCU option

On Sun, Sep 21, 2014 at 12:49:54PM -0400, Pranith Kumar wrote:
> PREEMPT_RCU and TREE_PREEMPT_RCU serve the same function after TINY_PREEMPT_RCU
> has been removed. This patch removes CONFIG_PREEMPT_RCU and uses
> CONFIG_TREE_PREEMPT_RCU in its place.
> 
> Signed-off-by: Pranith Kumar <bobby.prani@...il.com>

I think it would actually make more sense to do this the other way
around, if at all: drop TREE_PREEMPT_RCU in favor of PREEMPT_RCU.  The
instances shown here don't care about *which* preemptible RCU
implementation the kernel uses, they just need to behave differently
when preemptible.  Let's also not assume that no new RCU implementation
will ever arise superceding TREE_PREEMPT_RCU.

- Josh Triplett

>  arch/xtensa/configs/s6105_defconfig                      |  2 +-
>  include/linux/init_task.h                                |  2 +-
>  include/linux/rcupdate.h                                 | 16 ++++++++--------
>  include/linux/sched.h                                    | 10 +++++-----
>  init/Kconfig                                             |  6 ------
>  kernel/rcu/update.c                                      |  4 ++--
>  tools/testing/selftests/rcutorture/doc/TINY_RCU.txt      |  2 +-
>  .../selftests/rcutorture/doc/TREE_RCU-kconfig.txt        |  2 +-
>  8 files changed, 19 insertions(+), 25 deletions(-)
> 
> diff --git a/arch/xtensa/configs/s6105_defconfig b/arch/xtensa/configs/s6105_defconfig
> index 12a492a..dab1a99 100644
> --- a/arch/xtensa/configs/s6105_defconfig
> +++ b/arch/xtensa/configs/s6105_defconfig
> @@ -35,7 +35,7 @@ CONFIG_SYSVIPC_SYSCTL=y
>  #
>  # CONFIG_CLASSIC_RCU is not set
>  # CONFIG_TREE_RCU is not set
> -CONFIG_PREEMPT_RCU=y
> +CONFIG_TREE_PREEMPT_RCU=y
>  # CONFIG_RCU_TRACE is not set
>  # CONFIG_TREE_RCU_TRACE is not set
>  # CONFIG_PREEMPT_RCU_TRACE is not set
> diff --git a/include/linux/init_task.h b/include/linux/init_task.h
> index 77fc43f..39600f4 100644
> --- a/include/linux/init_task.h
> +++ b/include/linux/init_task.h
> @@ -108,7 +108,7 @@ extern struct group_info init_groups;
>  #else
>  #define INIT_TASK_RCU_TREE_PREEMPT(tsk)
>  #endif
> -#ifdef CONFIG_PREEMPT_RCU
> +#ifdef CONFIG_TREE_PREEMPT_RCU
>  #define INIT_TASK_RCU_PREEMPT(tsk)					\
>  	.rcu_read_lock_nesting = 0,					\
>  	.rcu_read_unlock_special.s = 0,					\
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index a4a819f..17a15d5 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -103,7 +103,7 @@ void do_trace_rcu_torture_read(const char *rcutorturename,
>  
>  /* Exported common interfaces */
>  
> -#ifdef CONFIG_PREEMPT_RCU
> +#ifdef CONFIG_TREE_PREEMPT_RCU
>  
>  /**
>   * call_rcu() - Queue an RCU callback for invocation after a grace period.
> @@ -140,12 +140,12 @@ void do_trace_rcu_torture_read(const char *rcutorturename,
>  void call_rcu(struct rcu_head *head,
>  	      void (*func)(struct rcu_head *head));
>  
> -#else /* #ifdef CONFIG_PREEMPT_RCU */
> +#else /* #ifdef CONFIG_TREE_PREEMPT_RCU */
>  
>  /* In classic RCU, call_rcu() is just call_rcu_sched(). */
>  #define	call_rcu	call_rcu_sched
>  
> -#endif /* #else #ifdef CONFIG_PREEMPT_RCU */
> +#endif /* #else #ifdef CONFIG_TREE_PREEMPT_RCU */
>  
>  /**
>   * call_rcu_bh() - Queue an RCU for invocation after a quicker grace period.
> @@ -217,7 +217,7 @@ void call_rcu_tasks(struct rcu_head *head, void (*func)(struct rcu_head *head));
>  void synchronize_rcu_tasks(void);
>  void rcu_barrier_tasks(void);
>  
> -#ifdef CONFIG_PREEMPT_RCU
> +#ifdef CONFIG_TREE_PREEMPT_RCU
>  
>  void __rcu_read_lock(void);
>  void __rcu_read_unlock(void);
> @@ -227,12 +227,12 @@ void synchronize_rcu(void);
>  /*
>   * Defined as a macro as it is a very low level header included from
>   * areas that don't even know about current.  This gives the rcu_read_lock()
> - * nesting depth, but makes sense only if CONFIG_PREEMPT_RCU -- in other
> + * nesting depth, but makes sense only if CONFIG_TREE_PREEMPT_RCU -- in other
>   * types of kernel builds, the rcu_read_lock() nesting depth is unknowable.
>   */
>  #define rcu_preempt_depth() (current->rcu_read_lock_nesting)
>  
> -#else /* #ifdef CONFIG_PREEMPT_RCU */
> +#else /* #ifdef CONFIG_TREE_PREEMPT_RCU */
>  
>  static inline void __rcu_read_lock(void)
>  {
> @@ -254,7 +254,7 @@ static inline int rcu_preempt_depth(void)
>  	return 0;
>  }
>  
> -#endif /* #else #ifdef CONFIG_PREEMPT_RCU */
> +#endif /* #else #ifdef CONFIG_TREE_PREEMPT_RCU */
>  
>  /* Internal to kernel */
>  void rcu_init(void);
> @@ -530,7 +530,7 @@ static inline int rcu_read_lock_sched_held(void)
>  		}							\
>  	} while (0)
>  
> -#if defined(CONFIG_PROVE_RCU) && !defined(CONFIG_PREEMPT_RCU)
> +#if defined(CONFIG_PROVE_RCU) && !defined(CONFIG_TREE_PREEMPT_RCU)
>  static inline void rcu_preempt_sleep_check(void)
>  {
>  	rcu_lockdep_assert(!lock_is_held(&rcu_lock_map),
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 42888d7..6acaa30 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1269,11 +1269,11 @@ struct task_struct {
>  	int nr_cpus_allowed;
>  	cpumask_t cpus_allowed;
>  
> -#ifdef CONFIG_PREEMPT_RCU
> +#ifdef CONFIG_TREE_PREEMPT_RCU
>  	int rcu_read_lock_nesting;
>  	union rcu_special rcu_read_unlock_special;
>  	struct list_head rcu_node_entry;
> -#endif /* #ifdef CONFIG_PREEMPT_RCU */
> +#endif /* #ifdef CONFIG_TREE_PREEMPT_RCU */
>  #ifdef CONFIG_TREE_PREEMPT_RCU
>  	struct rcu_node *rcu_blocked_node;
>  #endif /* #ifdef CONFIG_TREE_PREEMPT_RCU */
> @@ -2014,12 +2014,12 @@ extern void task_clear_jobctl_pending(struct task_struct *task,
>  
>  static inline void rcu_copy_process(struct task_struct *p)
>  {
> -#ifdef CONFIG_PREEMPT_RCU
> +#ifdef CONFIG_TREE_PREEMPT_RCU
>  	p->rcu_read_lock_nesting = 0;
>  	p->rcu_read_unlock_special.s = 0;
>  	p->rcu_blocked_node = NULL;
>  	INIT_LIST_HEAD(&p->rcu_node_entry);
> -#endif /* #ifdef CONFIG_PREEMPT_RCU */
> +#endif /* #ifdef CONFIG_TREE_PREEMPT_RCU */
>  #ifdef CONFIG_TASKS_RCU
>  	p->rcu_tasks_holdout = false;
>  	INIT_LIST_HEAD(&p->rcu_tasks_holdout_list);
> @@ -2752,7 +2752,7 @@ extern int __cond_resched_softirq(void);
>  
>  static inline void cond_resched_rcu(void)
>  {
> -#if defined(CONFIG_DEBUG_ATOMIC_SLEEP) || !defined(CONFIG_PREEMPT_RCU)
> +#if defined(CONFIG_DEBUG_ATOMIC_SLEEP) || !defined(CONFIG_TREE_PREEMPT_RCU)
>  	rcu_read_unlock();
>  	cond_resched();
>  	rcu_read_lock();
> diff --git a/init/Kconfig b/init/Kconfig
> index 8a2b97c..27ad03c 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -501,12 +501,6 @@ config TINY_RCU
>  
>  endchoice
>  
> -config PREEMPT_RCU
> -	def_bool TREE_PREEMPT_RCU
> -	help
> -	  This option enables preemptible-RCU code that is common between
> -	  TREE_PREEMPT_RCU and, in the old days, TINY_PREEMPT_RCU.
> -
>  config TASKS_RCU
>  	bool "Task_based RCU implementation using voluntary context switch"
>  	default n
> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> index 3ef8ba5..8848a32 100644
> --- a/kernel/rcu/update.c
> +++ b/kernel/rcu/update.c
> @@ -62,7 +62,7 @@ MODULE_ALIAS("rcupdate");
>  
>  module_param(rcu_expedited, int, 0);
>  
> -#ifdef CONFIG_PREEMPT_RCU
> +#ifdef CONFIG_TREE_PREEMPT_RCU
>  
>  /*
>   * Preemptible RCU implementation for rcu_read_lock().
> @@ -108,7 +108,7 @@ void __rcu_read_unlock(void)
>  }
>  EXPORT_SYMBOL_GPL(__rcu_read_unlock);
>  
> -#endif /* #ifdef CONFIG_PREEMPT_RCU */
> +#endif /* #ifdef CONFIG_TREE_PREEMPT_RCU */
>  
>  #ifdef CONFIG_DEBUG_LOCK_ALLOC
>  static struct lock_class_key rcu_lock_key;
> diff --git a/tools/testing/selftests/rcutorture/doc/TINY_RCU.txt b/tools/testing/selftests/rcutorture/doc/TINY_RCU.txt
> index 28db67b..4fea92d 100644
> --- a/tools/testing/selftests/rcutorture/doc/TINY_RCU.txt
> +++ b/tools/testing/selftests/rcutorture/doc/TINY_RCU.txt
> @@ -31,7 +31,7 @@ CONFIG_RCU_TORTURE_TEST
>  
>  CONFIG_HOTPLUG_CPU
>  CONFIG_PREEMPT
> -CONFIG_PREEMPT_RCU
> +CONFIG_TREE_PREEMPT_RCU
>  CONFIG_SMP
>  CONFIG_TINY_RCU
>  CONFIG_TREE_PREEMPT_RCU
> diff --git a/tools/testing/selftests/rcutorture/doc/TREE_RCU-kconfig.txt b/tools/testing/selftests/rcutorture/doc/TREE_RCU-kconfig.txt
> index f85c45e..081dece 100644
> --- a/tools/testing/selftests/rcutorture/doc/TREE_RCU-kconfig.txt
> +++ b/tools/testing/selftests/rcutorture/doc/TREE_RCU-kconfig.txt
> @@ -62,7 +62,7 @@ CONFIG_NO_HZ_FULL_SYSIDLE_SMALL
>  	Defer until Frederic uses this.
>  
>  CONFIG_PREEMPT_COUNT
> -CONFIG_PREEMPT_RCU
> +CONFIG_TREE_PREEMPT_RCU
>  
>  	Redundant with CONFIG_PREEMPT, ignore.
>  
> -- 
> 2.1.0
> 
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