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

[Paul E. McKenney]

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?

							Thanx, Paul

> Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> ---
>  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
>