linux-kernel - Re: [PATCH rcu 3/6] rcu/exp: Remove superfluous full memory barrier upon first EQS snapshot

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Date: Wed, 12 Jun 2024 14:14:14 +0530
From: Neeraj upadhyay <neeraj.iitr10@...il.com>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org, kernel-team@...a.com, 
	rostedt@...dmis.org, Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [PATCH rcu 3/6] rcu/exp: Remove superfluous full memory barrier
 upon first EQS snapshot

On Wed, Jun 5, 2024 at 3:58 AM Paul E. McKenney <paulmck@...nel.org> wrote:
>
> From: Frederic Weisbecker <frederic@...nel.org>
>
> When the grace period kthread checks the extended quiescent state
> counter of a CPU, full ordering is necessary to ensure that either:
>
> * If the GP kthread observes the remote target in an extended quiescent
>   state, then that target must observe all accesses prior to the current
>   grace period, including the current grace period sequence number, once
>   it exits that extended quiescent state.
>
> or:
>
> * If the GP kthread observes the remote target NOT in an extended
>   quiescent state, then the target further entering in an extended
>   quiescent state must observe all accesses prior to the current
>   grace period, including the current grace period sequence number, once
>   it enters that extended quiescent state.
>
> This ordering is enforced through a full memory barrier placed right
> before taking the first EQS snapshot. However this is superfluous
> because the snapshot is taken while holding the target's rnp lock which
> provides the necessary ordering through its chain of
> smp_mb__after_unlock_lock().
>
> Remove the needless explicit barrier before the snapshot and put a
> comment about the implicit barrier newly relied upon here.
>
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> ---
>  kernel/rcu/tree_exp.h | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index 8a1d9c8bd9f74..bec24ea6777e8 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -357,7 +357,13 @@ static void __sync_rcu_exp_select_node_cpus(struct rcu_exp_work *rewp)
>                     !(rnp->qsmaskinitnext & mask)) {
>                         mask_ofl_test |= mask;
>                 } else {
> -                       snap = rcu_dynticks_snap(cpu);
> +                       /*
> +                        * Full ordering against accesses prior current GP and
> +                        * also against current GP sequence number is enforced
> +                        * by current rnp locking with chained
> +                        * smp_mb__after_unlock_lock().

Again, worth mentioning the chaining sites sync_exp_reset_tree() and
this function?


Thanks
Neeraj

> +                        */
> +                       snap = ct_dynticks_cpu_acquire(cpu);
>                         if (rcu_dynticks_in_eqs(snap))
>                                 mask_ofl_test |= mask;
>                         else
> --
> 2.40.1
>
>