Re: [PATCH v9 6/7] rcu/tree: segcblist: Remove redundant smp_mb()s

Joel Fernandes
2020-11-07T00:26:08Z

On Wed, Nov 04, 2020 at 07:57:13PM -0800, Paul E. McKenney wrote:
> On Tue, Nov 03, 2020 at 09:26:02AM -0500, Joel Fernandes (Google) wrote:
> > This memory barrier is not needed as rcu_segcblist_add_len() already
> > includes a memory barrier *before* and *after* the length of the list is
> > updated.
> >
> > Same reasoning for rcu_segcblist_enqueue().
>
> I suggest a commit log like the following:
>
> ------------------------------------------------------------------------
>
> The full memory barriers in rcu_segcblist_enqueue() and in rcu_do_batch()
> are not needed because rcu_segcblist_add_len(), and thus also
> rcu_segcblist_inc_len(), already includes a memory barrier *before*
> and *after* the length of the list is updated.
>
> This commit therefore removes these redundant smp_mb() invocations.
>
> ------------------------------------------------------------------------
>
> Other than that, looks good! I could hand-apply it, but that
> would just cause more churn with the addition of the call to
> rcu_segcblist_inc_seglen(). So could you please update the commit log
> when you repost, whether to the mailing list or from your git tree?

Done, I updated it in my tree. I will share the link to tree on IRC.

thanks,

- Joel

>
> Thanx, Paul
>
> > Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
> > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > ---
> > kernel/rcu/rcu_segcblist.c | 1 -
> > kernel/rcu/tree.c | 1 -
> > 2 files changed, 2 deletions(-)
> >
> > diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
> > index e9e72d72f7a6..d96272e8d604 100644
> > --- a/kernel/rcu/rcu_segcblist.c
> > +++ b/kernel/rcu/rcu_segcblist.c
> > @@ -268,7 +268,6 @@ void rcu_segcblist_enqueue(struct rcu_segcblist *rsclp,
> > struct rcu_head *rhp)
> > {
> > rcu_segcblist_inc_len(rsclp);
> > - smp_mb(); /* Ensure counts are updated before callback is enqueued. */
> > rcu_segcblist_inc_seglen(rsclp, RCU_NEXT_TAIL);
> > rhp->next = NULL;
> > WRITE_ONCE(*rsclp->tails[RCU_NEXT_TAIL], rhp);
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index f6c6653b3ec2..fb2a5ac4a59c 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -2525,7 +2525,6 @@ static void rcu_do_batch(struct rcu_data *rdp)
> >
> > /* Update counts and requeue any remaining callbacks. */
> > rcu_segcblist_insert_done_cbs(&rdp->cblist, &rcl);
> > - smp_mb(); /* List handling before counting for rcu_barrier(). */
> > rcu_segcblist_add_len(&rdp->cblist, -count);
> >
> > /* Reinstate batch limit if we have worked down the excess. */
> > --
> > 2.29.1.341.ge80a0c044ae-goog
> >