Re: [PATCH v8 5/6] rcu/tree: segcblist: Remove redundant smp_mb()s

[Frederic Weisbecker <frederic@kernel.org>]

65;6003;1cOn Wed, Oct 21, 2020 at 03:08:12PM -0400, Joel Fernandes (Google) wrote:
> This memory barrier is not needed as rcu_segcblist_add_len() already
> includes a memory barrier *before* the length of the list is updated.

*before* and *after*.

As you have both cases below.

Thanks

> 
> Same reasoning for rcu_segcblist_enqueue().
> 
> 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 19ff82b805fb..f0fcdf9d0f7f 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 346a05506935..6c6d3c7036e6 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. */
> -- 
> 2.29.0.rc1.297.gfa9743e501-goog
>