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SubjectRe: [PATCH] rcu/tree: Add comment to describe GP done condition in fqs loop
On Fri, Jun 10, 2022 at 08:45:42AM +0530, Neeraj Upadhyay wrote:
>
>
> 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"?

And the winner is "the corresponding leaf nodes"! Good catch, and
thank you!

Thanx, Paul

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

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