Re: [PATCH tip/core/rcu 02/15] rcu: Pull TINY_RCU dyntick-idle tracing into non-idle region

[Josh Triplett <josh@joshtriplett.org>]

On Thu, Aug 30, 2012 at 11:56:15AM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paul.mckenney@linaro.org>
> 
> Because TINY_RCU's idle detection keys directly off of the nesting
> level, rather than from a separate variable as in TREE_RCU, the
> TINY_RCU dyntick-idle tracing on transition to idle must happen
> before the change to the nesting level.  This commit therefore makes
> this change by passing the desired new value (rather than the old value)
> of the nesting level in to rcu_idle_enter_common().
> 
> [ paulmck: Add fix for wrong-variable bug spotted by
>   Michael Wang <wangyun@linux.vnet.ibm.com>. ]
> 
> Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Reviewed-by: Josh Triplett <josh@joshtriplett.org>

> ---
>  kernel/rcutiny.c |   31 ++++++++++++++++---------------
>  1 files changed, 16 insertions(+), 15 deletions(-)
> 
> diff --git a/kernel/rcutiny.c b/kernel/rcutiny.c
> index 547b1fe..e4163c5 100644
> --- a/kernel/rcutiny.c
> +++ b/kernel/rcutiny.c
> @@ -56,24 +56,27 @@ static void __call_rcu(struct rcu_head *head,
>  static long long rcu_dynticks_nesting = DYNTICK_TASK_EXIT_IDLE;
>  
>  /* Common code for rcu_idle_enter() and rcu_irq_exit(), see kernel/rcutree.c. */
> -static void rcu_idle_enter_common(long long oldval)
> +static void rcu_idle_enter_common(long long newval)
>  {
> -	if (rcu_dynticks_nesting) {
> +	if (newval) {
>  		RCU_TRACE(trace_rcu_dyntick("--=",
> -					    oldval, rcu_dynticks_nesting));
> +					    rcu_dynticks_nesting, newval));
> +		rcu_dynticks_nesting = newval;
>  		return;
>  	}
> -	RCU_TRACE(trace_rcu_dyntick("Start", oldval, rcu_dynticks_nesting));
> +	RCU_TRACE(trace_rcu_dyntick("Start", rcu_dynticks_nesting, newval));
>  	if (!is_idle_task(current)) {
>  		struct task_struct *idle = idle_task(smp_processor_id());
>  
>  		RCU_TRACE(trace_rcu_dyntick("Error on entry: not idle task",
> -					    oldval, rcu_dynticks_nesting));
> +					    rcu_dynticks_nesting, newval));
>  		ftrace_dump(DUMP_ALL);
>  		WARN_ONCE(1, "Current pid: %d comm: %s / Idle pid: %d comm: %s",
>  			  current->pid, current->comm,
>  			  idle->pid, idle->comm); /* must be idle task! */
>  	}
> +	barrier();
> +	rcu_dynticks_nesting = newval;
>  	rcu_sched_qs(0); /* implies rcu_bh_qsctr_inc(0) */
>  }
>  
> @@ -84,17 +87,16 @@ static void rcu_idle_enter_common(long long oldval)
>  void rcu_idle_enter(void)
>  {
>  	unsigned long flags;
> -	long long oldval;
> +	long long newval;
>  
>  	local_irq_save(flags);
> -	oldval = rcu_dynticks_nesting;
>  	WARN_ON_ONCE((rcu_dynticks_nesting & DYNTICK_TASK_NEST_MASK) == 0);
>  	if ((rcu_dynticks_nesting & DYNTICK_TASK_NEST_MASK) ==
>  	    DYNTICK_TASK_NEST_VALUE)
> -		rcu_dynticks_nesting = 0;
> +		newval = 0;
>  	else
> -		rcu_dynticks_nesting  -= DYNTICK_TASK_NEST_VALUE;
> -	rcu_idle_enter_common(oldval);
> +		newval = rcu_dynticks_nesting - DYNTICK_TASK_NEST_VALUE;
> +	rcu_idle_enter_common(newval);
>  	local_irq_restore(flags);
>  }
>  EXPORT_SYMBOL_GPL(rcu_idle_enter);
> @@ -105,13 +107,12 @@ EXPORT_SYMBOL_GPL(rcu_idle_enter);
>  void rcu_irq_exit(void)
>  {
>  	unsigned long flags;
> -	long long oldval;
> +	long long newval;
>  
>  	local_irq_save(flags);
> -	oldval = rcu_dynticks_nesting;
> -	rcu_dynticks_nesting--;
> -	WARN_ON_ONCE(rcu_dynticks_nesting < 0);
> -	rcu_idle_enter_common(oldval);
> +	newval = rcu_dynticks_nesting - 1;
> +	WARN_ON_ONCE(newval < 0);
> +	rcu_idle_enter_common(newval);
>  	local_irq_restore(flags);
>  }
>  
> -- 
> 1.7.8
>