linux-kernel - Re: [PATCH 2/9] rcu: Use rcu_segcblist_segempty() instead of open coding it

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Date:   Fri, 2 Jun 2023 19:00:00 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     "Paul E . McKenney" <paulmck@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, rcu <rcu@...r.kernel.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Giovanni Gherdovich <ggherdovich@...e.cz>
Subject: Re: [PATCH 2/9] rcu: Use rcu_segcblist_segempty() instead of open
 coding it

On Wed, May 31, 2023 at 6:17 AM Frederic Weisbecker <frederic@...nel.org> wrote:
>
> This makes the code more readable.
>
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> ---
>  kernel/rcu/rcu_segcblist.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
> index f71fac422c8f..1693ea22ef1b 100644
> --- a/kernel/rcu/rcu_segcblist.c
> +++ b/kernel/rcu/rcu_segcblist.c
> @@ -368,7 +368,7 @@ bool rcu_segcblist_entrain(struct rcu_segcblist *rsclp,
>         smp_mb(); /* Ensure counts are updated before callback is entrained. */
>         rhp->next = NULL;
>         for (i = RCU_NEXT_TAIL; i > RCU_DONE_TAIL; i--)
> -               if (rsclp->tails[i] != rsclp->tails[i - 1])
> +               if (!rcu_segcblist_segempty(rsclp, i))

Hopefully the compiler optimizer will be smart enough to remove this
from the inlined code ;-):
if (seg == RCU_DONE_TAIL)
   return &rsclp->head == rsclp->tails[RCU_DONE_TAIL];

Otherwise it appears to be no functional change for this and the below
change, and straightforward so for this patch:
Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>

thanks,

 - Joel


>                         break;
>         rcu_segcblist_inc_seglen(rsclp, i);
>         WRITE_ONCE(*rsclp->tails[i], rhp);
> @@ -551,7 +551,7 @@ bool rcu_segcblist_accelerate(struct rcu_segcblist *rsclp, unsigned long seq)
>          * as their ->gp_seq[] grace-period completion sequence number.
>          */
>         for (i = RCU_NEXT_READY_TAIL; i > RCU_DONE_TAIL; i--)
> -               if (rsclp->tails[i] != rsclp->tails[i - 1] &&
> +               if (!rcu_segcblist_segempty(rsclp, i) &&
>                     ULONG_CMP_LT(rsclp->gp_seq[i], seq))
>                         break;
>
> --
> 2.40.1
>