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

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