linux-kernel - Re: [PATCH 3/4] rcu: Use better variable names in funnel locking loop

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Message-Id: <20180523192349.GA5845@linux.vnet.ibm.com>
Date:   Wed, 23 May 2018 12:23:49 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Joel Fernandes <joelaf@...gle.com>
Cc:     linux-kernel@...r.kernel.org,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Boqun Feng <boqun.feng@...il.com>, byungchul.park@....com,
        Ingo Molnar <mingo@...hat.com>,
        Josh Triplett <josh@...htriplett.org>, kernel-team@...roid.com,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Peter Zilstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 3/4] rcu: Use better variable names in funnel locking loop

On Wed, May 23, 2018 at 09:06:17AM -0700, Paul E. McKenney wrote:
> On Tue, May 22, 2018 at 11:38:14PM -0700, Joel Fernandes wrote:
> > From: "Joel Fernandes (Google)" <joel@...lfernandes.org>
> > 
> > The funnel locking loop in rcu_start_this_gp uses rcu_root as a
> > temporary variable while walking the combining tree. This causes a
> > tiresome exercise of a code reader reminding themselves that rcu_root
> > may not be root. Lets just call it rnp, and rename other variables as
> > well to be more appropriate.
> > 
> > Original patch: https://patchwork.kernel.org/patch/10396577/
> > 
> > Signed-off-by: Joel Fernandes <joel@...lfernandes.org>
> > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> 
> I used to have double Signed-off-by back when I was seconded to Linaro.
> But I am guessing that you want the second and don't need the first
> one.  Unless you tell me otherwise, I will remove the first one on
> my next rebase.
> 
> Anyway, the new variable names are much more clear, good stuff,
> queued for further review and testing, thank you!

And it looks to me like I should fold in the patchlet below to change to
rnp_start in a comment.  Please let me know if this would mess things up.

							Thanx, Paul

------------------------------------------------------------------------

commit 94ce05d9d110b8c34eca6641ca5221c1b150e99f
Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Date:   Wed May 23 12:22:01 2018 -0700

    fixup! rcu: Use better variable names in funnel locking loop
    
    Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 65e49282429c..fdba8ab95e2c 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1550,11 +1550,11 @@ static bool rcu_start_this_gp(struct rcu_node *rnp_start, struct rcu_data *rdp,
 	/*
 	 * Use funnel locking to either acquire the root rcu_node
 	 * structure's lock or bail out if the need for this grace period
-	 * has already been recorded -- or has already started.  If there
-	 * is already a grace period in progress in a non-leaf node, no
-	 * recording is needed because the end of the grace period will
-	 * scan the leaf rcu_node structures.  Note that rnp->lock must
-	 * not be released.
+	 * has already been recorded -- or if that grace period has in
+	 * fact already started.  If there is already a grace period in
+	 * progress in a non-leaf node, no recording is needed because the
+	 * end of the grace period will scan the leaf rcu_node structures.
+	 * Note that rnp_start->lock must not be released.
 	 */
 	raw_lockdep_assert_held_rcu_node(rnp_start);
 	trace_rcu_this_gp(rnp_start, rdp, gp_seq_req, TPS("Startleaf"));