Re: [PATCH] rcu/tree: Add comment to describe GP done condition in fqs loop

[Neeraj Upadhyay <quic_neeraju@quicinc.com>]

On 6/10/2022 1:53 AM, Paul E. McKenney wrote:
> On Thu, Jun 09, 2022 at 12:43:40PM +0530, Neeraj Upadhyay wrote:
>> Add a comment to explain why !rcu_preempt_blocked_readers_cgp() condition
>> is required on root rnp node, for GP completion check in rcu_gp_fqs_loop().
>>
>> Signed-off-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
> 
> Thank you, Neeraj!  As usual, I could not resist the urge to wordsmith
> as shown below.  Could you please check to see if I messed something up?

Thanks!
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> commit bdf3a744d3ad21336a390bfcc2e41de63f193eaf
> Author: Neeraj Upadhyay <quic_neeraju@quicinc.com>
> Date:   Thu Jun 9 12:43:40 2022 +0530
> 
>      rcu/tree: Add comment to describe GP-done condition in fqs loop
>      
>      Add a comment to explain why !rcu_preempt_blocked_readers_cgp() condition
>      is required on root rnp node, for GP completion check in rcu_gp_fqs_loop().
>      
>      Signed-off-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
>      Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index a93c5f4d7e092..9a941e7ee6109 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2083,7 +2083,15 @@ static noinline_for_stack void rcu_gp_fqs_loop(void)
>   		rcu_gp_torture_wait();
>   		WRITE_ONCE(rcu_state.gp_state, RCU_GP_DOING_FQS);
>   		/* Locking provides needed memory barriers. */
> -		/* If grace period done, leave loop. */
> +		/*
> +		 * Exit the loop if the root rcu_node structure indicates that the grace period
> +		 * has ended, leave the loop.  The rcu_preempt_blocked_readers_cgp(rnp) check

We can remove ", leave the loop" ?

> +		 * is required only for single-node rcu_node trees because readers blocking
> +		 * the current grace period are queued only on leaf rcu_node structures.
> +		 * For multi-node trees, checking the root node's ->qsmask suffices, because a
> +		 * given root node's ->qsmask bit is cleared only when all CPUs and tasks from

Do we need to say "a given root node's" , we have only single RCU node 
in the system, so we can just say, "because root node's ->qsmask bit is 
cleared..." ?

> +		 * the corresponding leaf node have passed through their quiescent state.

Change "the corresponding leaf node" to "their corresponding leaf nodes" 
or "the corresponding leaf nodes"?


Thanks
Neeraj

> +		 */
>   		if (!READ_ONCE(rnp->qsmask) &&
>   		    !rcu_preempt_blocked_readers_cgp(rnp))
>   			break;