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

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Message-ID: <20220610034135.GA1790663@paulmck-ThinkPad-P17-Gen-1>
Date:   Thu, 9 Jun 2022 20:41:35 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Neeraj Upadhyay <quic_neeraju@...cinc.com>
Cc:     frederic@...nel.org, josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        joel@...lfernandes.org, linux-kernel@...r.kernel.org,
        boqun.feng@...il.com, urezki@...il.com
Subject: Re: [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@...cinc.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@...cinc.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@...cinc.com>
> >      Signed-off-by: Paul E. McKenney <paulmck@...nel.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;