Re: [RFC] rcu: Prevent expedite reporting within RCU read-side section

[Boqun Feng]

On Tue, Mar 06, 2018 at 02:31:58PM +0900, Byungchul Park wrote:
> Hello Paul and RCU folks,
> 
> I am afraid I correctly understand and fix it. But I really wonder why
> sync_rcu_exp_handler() reports the quiescent state even in the case that
> current task is within a RCU read-side section. Do I miss something?
> 
> If I correctly understand it and you agree with it, I can add more logic
> which make it more expedited by boosting current or making it urgent
> when we fail to report the quiescent state on the IPI.
> 
> ----->8-----
> From 0b0191f506c19ce331a1fdb7c2c5a00fb23fbcf2 Mon Sep 17 00:00:00 2001
> From: Byungchul Park <byungchul.park@xxxxxxx>
> Date: Tue, 6 Mar 2018 13:54:41 +0900
> Subject: [RFC] rcu: Prevent expedite reporting within RCU read-side section
> 
> We report the quiescent state for this cpu if it's out of RCU read-side
> section at the moment IPI was just fired during the expedite process.
> 
> However, current code reports the quiescent state even in the case:
> 
>    1) the current task is still within a RCU read-side section
>    2) the current task has been blocked within the RCU read-side section
> 

If this happens, the task will queue itself in
rcu_preempt_note_context_switch() using rcu_preempt_ctxt_queue(). The gp
kthread will wait for this task to dequeue itself. IOW, we have other
mechanism to wait for this task other than bottom-up qs reporting tree.
So I think we are fine here.

Regards,
Boqun

> Since we don't get to the quiescent state yet in the case, we shouldn't
> report it but check it another time.
> 
> Signed-off-by: Byungchul Park <byungchul.park@xxxxxxx>
> ---
>  kernel/rcu/tree_exp.h | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index 73e1d3d..cc69d14 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -731,13 +731,13 @@ static void sync_rcu_exp_handler(void *info)
>  	/*
>  	 * We are either exiting an RCU read-side critical section (negative
>  	 * values of t->rcu_read_lock_nesting) or are not in one at all
> -	 * (zero value of t->rcu_read_lock_nesting).  Or we are in an RCU
> -	 * read-side critical section that blocked before this expedited
> -	 * grace period started.  Either way, we can immediately report
> -	 * the quiescent state.
> +	 * (zero value of t->rcu_read_lock_nesting). We can immediately
> +	 * report the quiescent state.
>  	 */
> -	rdp = this_cpu_ptr(rsp->rda);
> -	rcu_report_exp_rdp(rsp, rdp, true);
> +	if (t->rcu_read_lock_nesting <= 0) {
> +		rdp = this_cpu_ptr(rsp->rda);
> +		rcu_report_exp_rdp(rsp, rdp, true);
> +	}
>  }
>  
>  /**
> -- 
> 1.9.1
>

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