Re: [PATCH v9 2/7] rcu/segcblist: Add counters to segcblist datastructure

Paul E. McKenney
2020-11-07T00:48:18Z

On Fri, Nov 06, 2020 at 07:18:47PM -0500, Joel Fernandes wrote:
> On Fri, Nov 06, 2020 at 07:01:57PM -0500, Joel Fernandes wrote:
> > On Wed, Nov 04, 2020 at 09:01:33AM -0800, Paul E. McKenney wrote:
> >
> > > A casual reader might be forgiven for being confused by the combination
> > > of "Return" in the above comment and the "void" function type below.
> > > So shouldn't this comment be something like "Add the specified number
> > > of callbacks to the specified segment..."?
> >
> > You are right, sorry and will fix it.
> >
> > > > @@ -330,11 +342,16 @@ void rcu_segcblist_extract_pend_cbs(struct rcu_segcblist *rsclp,
> > > >
> > > > if (!rcu_segcblist_pend_cbs(rsclp))
> > > > return; /* Nothing to do. */
> > > > + rclp->len = rcu_segcblist_get_seglen(rsclp, RCU_WAIT_TAIL) +
> > > > + rcu_segcblist_get_seglen(rsclp, RCU_NEXT_READY_TAIL) +
> > > > + rcu_segcblist_get_seglen(rsclp, RCU_NEXT_TAIL);
> > >
> > > This should be a "for" loop. Yes, the number and names of the segments
> > > hasn't changed for a good long time, but nothing like code as above to
> > > inspire Murphy to more mischief. :-/
> > >
> > > Actually, why not put the summation in the existing "for" loop below?
> > > That would save a line of code in addition to providing less inspiration
> > > for Mr. Murphy.
> >
> > I can do that. Actually Frederic suggested the same thing but I was reluctant
> > as I felt it did not give much LOC benefit. Will revisit it.
>
> It reduces 1 line of code :) I changed it to the below, will update the patch:

Thank you! And yes, I am much more concerned about the constraints on
Mr. Murphy than on the lines of code. ;-)

Thanx, Paul

> ---8<-----------------------
>
> diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
> index 9b43d686b1f3..bff9b2253e50 100644
> --- a/kernel/rcu/rcu_segcblist.c
> +++ b/kernel/rcu/rcu_segcblist.c
> @@ -101,7 +101,7 @@ static void rcu_segcblist_set_seglen(struct rcu_segcblist *rsclp, int seg, long
> WRITE_ONCE(rsclp->seglen[seg], v);
> }
>
> -/* Return number of callbacks in a segment of the segmented callback list. */
> +/* Increase the numeric length of a segment by a specified amount. */
> static void rcu_segcblist_add_seglen(struct rcu_segcblist *rsclp, int seg, long v)
> {
> WRITE_ONCE(rsclp->seglen[seg], rsclp->seglen[seg] + v);
> @@ -406,13 +406,12 @@ void rcu_segcblist_extract_pend_cbs(struct rcu_segcblist *rsclp,
>
> if (!rcu_segcblist_pend_cbs(rsclp))
> return; /* Nothing to do. */
> - rclp->len = rcu_segcblist_get_seglen(rsclp, RCU_WAIT_TAIL) +
> - rcu_segcblist_get_seglen(rsclp, RCU_NEXT_READY_TAIL) +
> - rcu_segcblist_get_seglen(rsclp, RCU_NEXT_TAIL);
> + rclp->len = 0;
> *rclp->tail = *rsclp->tails[RCU_DONE_TAIL];
> rclp->tail = rsclp->tails[RCU_NEXT_TAIL];
> WRITE_ONCE(*rsclp->tails[RCU_DONE_TAIL], NULL);
> for (i = RCU_DONE_TAIL + 1; i < RCU_CBLIST_NSEGS; i++) {
> + rclp->len += rcu_segcblist_get_seglen(rsclp, i);
> WRITE_ONCE(rsclp->tails[i], rsclp->tails[RCU_DONE_TAIL]);
> rcu_segcblist_set_seglen(rsclp, i, 0);
> }