Re: [RFC][PATCH] rcu: Clean up TASKS_RCU() abuse

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Nov 11, 2015 at 01:23:10PM +0100, Peter Zijlstra wrote:
> Hi
> 
> I recently ran into TASKS_RCU() and wondered why we can't use normal
> coding patterns to do the same.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Well, I cannot get too excited either way, but the diffstat for this
change is not particularly favorable.

							Thanx, Paul

> ---
>  include/linux/rcupdate.h | 33 +++++++++++++++++++++++++++++++--
>  kernel/exit.c            |  9 +++------
>  2 files changed, 34 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index a0189ba67fde..15a82372b372 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -389,7 +389,7 @@ static inline void rcu_init_nohz(void)
>   * macro rather than an inline function to avoid #include hell.
>   */
>  #ifdef CONFIG_TASKS_RCU
> -#define TASKS_RCU(x) x
> +
>  extern struct srcu_struct tasks_rcu_exit_srcu;
>  #define rcu_note_voluntary_context_switch(t) \
>  	do { \
> @@ -397,9 +397,38 @@ extern struct srcu_struct tasks_rcu_exit_srcu;
>  		if (READ_ONCE((t)->rcu_tasks_holdout)) \
>  			WRITE_ONCE((t)->rcu_tasks_holdout, false); \
>  	} while (0)
> +
> +static inline int tasks_rcu_read_lock(void)
> +{
> +	int idx;
> +
> +	preempt_disable();
> +	idx = __srcu_read_lock(&tasks_rcu_exit_srcu);
> +	preempt_enable();
> +
> +	return idx;
> +}
> +
> +static inline void tasks_rcu_read_unlock(int idx)
> +{
> +	preempt_disable();
> +	__srcu_read_unlock(&tasks_rcu_exit_srcu, idx);
> +	preempt_enable();
> +}
> +
>  #else /* #ifdef CONFIG_TASKS_RCU */
> -#define TASKS_RCU(x) do { } while (0)
> +
>  #define rcu_note_voluntary_context_switch(t)	rcu_all_qs()
> +
> +static inline int tasks_rcu_read_lock(void)
> +{
> +	return 0;
> +}
> +
> +static inline void tasks_rcu_read_unlock(int idx)
> +{
> +}
> +
>  #endif /* #else #ifdef CONFIG_TASKS_RCU */
> 
>  /**
> diff --git a/kernel/exit.c b/kernel/exit.c
> index 07110c6020a0..cd5644baeb22 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -653,8 +653,7 @@ static inline void check_stack_usage(void) {}
>  void do_exit(long code)
>  {
>  	struct task_struct *tsk = current;
> -	int group_dead;
> -	TASKS_RCU(int tasks_rcu_i);
> +	int group_dead, tasks_rcu_i;
> 
>  	profile_task_exit(tsk);
> 
> @@ -763,9 +762,7 @@ void do_exit(long code)
>  	 */
>  	flush_ptrace_hw_breakpoint(tsk);
> 
> -	TASKS_RCU(preempt_disable());
> -	TASKS_RCU(tasks_rcu_i = __srcu_read_lock(&tasks_rcu_exit_srcu));
> -	TASKS_RCU(preempt_enable());
> +	tasks_rcu_i = tasks_rcu_read_lock();
>  	exit_notify(tsk, group_dead);
>  	proc_exit_connector(tsk);
>  #ifdef CONFIG_NUMA
> @@ -805,7 +802,7 @@ void do_exit(long code)
>  	if (tsk->nr_dirtied)
>  		__this_cpu_add(dirty_throttle_leaks, tsk->nr_dirtied);
>  	exit_rcu();
> -	TASKS_RCU(__srcu_read_unlock(&tasks_rcu_exit_srcu, tasks_rcu_i));
> +	tasks_rcu_read_unlock(tasks_rcu_i);
> 
>  	/*
>  	 * The setting of TASK_RUNNING by try_to_wake_up() may be delayed
>