linux-kernel - Re: [PATCH 2/2] rcu/tree: Clarify comments about FQS loop reporting quiescent states

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Message-ID: <20200731013443.GG9247@paulmck-ThinkPad-P72>
Date:   Thu, 30 Jul 2020 18:34:44 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        rcu <rcu@...r.kernel.org>, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 2/2] rcu/tree: Clarify comments about FQS loop reporting
 quiescent states

On Thu, Jul 30, 2020 at 09:21:52PM -0400, Joel Fernandes wrote:
> On Thu, Jul 30, 2020 at 09:35:20AM -0700, Paul E. McKenney wrote:
> > On Wed, Jul 29, 2020 at 11:25:19PM -0400, Joel Fernandes wrote:
> > > On Wed, Jul 29, 2020 at 11:02 PM Joel Fernandes (Google)
> > > <joel@...lfernandes.org> wrote:
> > > >
> > > > At least since v4.19, the FQS loop no longer reports quiescent states
> > > 
> > > I meant here, "FQS loop no longer reports quiescent states for offline CPUs."
> > > 
> > > Sorry,
> > 
> > You did have me going there for a bit.  ;-)
> > 
> > No period (".") at the end though, unless you fix up the following
> > to start a new sentence.
> 
> Ok.
> 
> > > > unless it is a dire situation where an offlined CPU failed to report
> > > > a quiescent state. Let us clarify the comment in rcu_gp_init() inorder
> > > > to keep the comment current.
> > 
> > How about the following for this last sentence?
> > 
> > "This commit therefore fixes the comment in rcu_gp_init() to match
> > the current code."
> 
> As per:
> https://www.kernel.org/doc/html/v4.17/process/submitting-patches.html
> 
> It says:
> Describe your changes in imperative mood, e.g. “make xyzzy do frotz” instead
> of “[This patch] makes xyzzy do frotz” or “[I] changed xyzzy to do frotz”, as
> if you are giving orders to the codebase to change its behaviour.
> 
> May be I should make it "Fix the comment in rcu_gp_init() to match the
> current code"?

What submitting-patches.rst is objecting to is starting the commit
log with "This patch...".  I am suggesting something quite different,
namely providing a clear indication of the transition from problem
statement to solution.

> > > > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > > > ---
> > > >  kernel/rcu/tree.c | 4 ++--
> > > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > > index 1e51962b565b..929568ff5989 100644
> > > > --- a/kernel/rcu/tree.c
> > > > +++ b/kernel/rcu/tree.c
> > > > @@ -1701,8 +1701,8 @@ static bool rcu_gp_init(void)
> > > >
> > > >         /*
> > > >          * Apply per-leaf buffered online and offline operations to the
> > > > -        * rcu_node tree.  Note that this new grace period need not wait
> > > > -        * for subsequent online CPUs, and that quiescent-state forcing
> > > > +        * rcu_node tree.  Note that this new grace period need not wait for
> > > > +        * subsequent online CPUs, and that RCU hooks in CPU offlining path
> > > >          * will handle subsequent offline CPUs.
> > 
> > How about something like this?
> > 
> > 	...  Note that this new grace period ned not wait for subsequent
> > 	online CPUs, and that RCU hooks in the CPU offlining path, when
> > 	combined with checks in this function, will handle CPUs that
> > 	are currently going offline and that go offline later.
> 
> Sounds good to me. I think s/and that go/or that go/ though.

Good point!  Another approach would be s/and that/and those that/
but yours works.

> I will make these changes and send v3, let me know though if you object.

Sounds good!

							Thanx, Paul

> thanks,
> 
>  - Joel
> 
> 
> > 						Thanx, Paul
> > 
> > > >          */
> > > >         rcu_state.gp_state = RCU_GP_ONOFF;
> > > > --
> > > > 2.28.0.rc0.142.g3c755180ce-goog
> > > >