linux-kernel - Re: [PATCH rcu 3/6 v2] rcu/exp: Remove superfluous full memory barrier upon first EQS snapshot

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Date: Thu, 27 Jun 2024 11:53:29 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org, kernel-team@...a.com,
	rostedt@...dmis.org
Subject: Re: [PATCH rcu 3/6 v2] rcu/exp: Remove superfluous full memory
 barrier upon first EQS snapshot

On Thu, Jun 27, 2024 at 01:36:58PM +0200, Frederic Weisbecker wrote:
> When the grace period kthread checks the extended quiescent state
> counter of a CPU, full ordering is necessary to ensure that either:

Just to make sure that I understand...

I need to replace these commits in -rcu:

da979d0162fc6 rcu: Remove full ordering on second EQS snapshot
6411f4185f657 rcu: Remove superfluous full memory barrier upon first EQS snapshot
dec56ca5f1c34 rcu/exp: Remove superfluous full memory barrier upon first EQS snapshot

With these three patches, and keep these three commits as they are?

d43a302fc08a5 rcu: Remove full memory barrier on boot time eqs sanity check
b1c36aa90cbf1 rcu: Remove full memory barrier on RCU stall printout
64d68f1d53f77 rcu/exp: Remove redundant full memory barrier at the end of GP

							Thanx, Paul

> * 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>
> Reviewed-by: Boqun Feng <boqun.feng@...il.com>
> Reviewed-by: Neeraj Upadhyay <neeraj.upadhyay@...nel.org>
> ---
>  kernel/rcu/tree_exp.h | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index 8a1d9c8bd9f7..1dbad2442e8d 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -357,7 +357,21 @@ 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 between remote CPU's post idle accesses
> +			 * and updater's accesses prior to current GP (and also
> +			 * the started GP sequence number) is enforced by
> +			 * rcu_seq_start() implicit barrier, relayed by kworkers
> +			 * locking and even further by smp_mb__after_unlock_lock()
> +			 * barriers chained all the way throughout the rnp locking
> +			 * tree since sync_exp_reset_tree() and up to the current
> +			 * leaf rnp locking.
> +			 *
> +			 * Ordering between remote CPU's pre idle accesses and
> +			 * post grace period updater's accesses is enforced by the
> +			 * below acquire semantic.
> +			 */
> +			snap = ct_dynticks_cpu_acquire(cpu);
>  			if (rcu_dynticks_in_eqs(snap))
>  				mask_ofl_test |= mask;
>  			else
> -- 
> 2.45.2
>