linux-kernel - Re: [PATCH 1/6] rcu: Remove full ordering on second EQS snapshot

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Date: Wed, 15 May 2024 19:32:45 +0200
From: Valentin Schneider <vschneid@...hat.com>
To: Frederic Weisbecker <frederic@...nel.org>, LKML
 <linux-kernel@...r.kernel.org>
Cc: Frederic Weisbecker <frederic@...nel.org>, "Paul E . McKenney"
 <paulmck@...nel.org>, Boqun Feng <boqun.feng@...il.com>, Joel Fernandes
 <joel@...lfernandes.org>, Neeraj Upadhyay <neeraj.upadhyay@....com>,
 Uladzislau Rezki <urezki@...il.com>, Zqiang <qiang.zhang1211@...il.com>,
 rcu <rcu@...r.kernel.org>
Subject: Re: [PATCH 1/6] rcu: Remove full ordering on second EQS snapshot

On 15/05/24 14:53, 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:
>
> * 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. Also the GP kthread must
>   observe all accesses performed by the target prior it entering in
>   EQS.
>
> 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. Also the GP kthread later
>   observing that EQS must also observe all accesses performed by the
>   target prior it entering in EQS.
>
> This ordering is explicitly performed both on the first EQS snapshot
> and on the second one as well through the combination of a preceding
> full barrier followed by an acquire read. However the second snapshot's
> full memory barrier is redundant and not needed to enforce the above
> guarantees:
>
>     GP kthread                  Remote target
>     ----                        -----
>     // Access prior GP
>     WRITE_ONCE(A, 1)
>     // first snapshot
>     smp_mb()
>     x = smp_load_acquire(EQS)
>                                // Access prior GP
>                                WRITE_ONCE(B, 1)
>                                // EQS enter
>                                // implied full barrier by atomic_add_return()
>                                atomic_add_return(RCU_DYNTICKS_IDX, EQS)
>                                // implied full barrier by atomic_add_return()
>                                READ_ONCE(A)
>     // second snapshot
>     y = smp_load_acquire(EQS)
>     z = READ_ONCE(B)
>
> If the GP kthread above fails to observe the remote target in EQS
> (x not in EQS), the remote target will observe A == 1 after further
> entering in EQS. Then the second snapshot taken by the GP kthread only
> need to be an acquire read in order to observe z == 1.
>
> Therefore remove the needless full memory barrier on second snapshot.
>
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>

Still looking at the rest, but at least so far I'm convinced this one makes
sense.

Reviewed-by: Valentin Schneider <vschneid@...hat.com>

> ---
>  kernel/rcu/tree.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 5e6828132007..58415cdc54f8 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -325,7 +325,7 @@ static bool rcu_dynticks_in_eqs(int snap)
>   */
>  static bool rcu_dynticks_in_eqs_since(struct rcu_data *rdp, int snap)
>  {
> -	return snap != rcu_dynticks_snap(rdp->cpu);
> +	return snap != ct_dynticks_cpu_acquire(rdp->cpu);
>  }
>
>  /*
> --
> 2.44.0