linux-kernel - Re: [PATCH 4/4] rcu: Name internal polling flag

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Message-ID: <20220322021107.GP4285@paulmck-ThinkPad-P17-Gen-1>
Date:   Mon, 21 Mar 2022 19:11:07 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Uladzislau Rezki <uladzislau.rezki@...y.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH 4/4] rcu: Name internal polling flag

On Wed, Mar 16, 2022 at 03:42:55PM +0100, Frederic Weisbecker wrote:
> Give a proper self-explanatory name to the expedited grace period
> internal polling flag.
> 
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> Cc: Neeraj Upadhyay <quic_neeraju@...cinc.com>
> Cc: Boqun Feng <boqun.feng@...il.com>
> Cc: Uladzislau Rezki <uladzislau.rezki@...y.com>
> Cc: Joel Fernandes <joel@...lfernandes.org>
> ---
>  kernel/rcu/rcu.h      | 5 +++++
>  kernel/rcu/tree.c     | 2 +-
>  kernel/rcu/tree_exp.h | 9 +++++----
>  3 files changed, 11 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
> index eccbdbdaa02e..8a62bb416ba4 100644
> --- a/kernel/rcu/rcu.h
> +++ b/kernel/rcu/rcu.h
> @@ -30,6 +30,11 @@
>  #define RCU_GET_STATE_USE_NORMAL	0x2
>  #define RCU_GET_STATE_BAD_FOR_NORMAL	(RCU_GET_STATE_FROM_EXPEDITED | RCU_GET_STATE_USE_NORMAL)
>  
> +/*
> + * Low-order bit definitions for polled grace-period internals.
> + */
> +#define RCU_EXP_SEQ_POLL_DONE 0x1
> +
>  /*
>   * Return the counter portion of a sequence number previously returned
>   * by rcu_seq_snap() or rcu_seq_current().
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 5da381a3cbe5..b3223b365f9f 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -4679,7 +4679,7 @@ static void __init rcu_init_one(void)
>  			spin_lock_init(&rnp->exp_lock);
>  			mutex_init(&rnp->boost_kthread_mutex);
>  			raw_spin_lock_init(&rnp->exp_poll_lock);
> -			rnp->exp_seq_poll_rq = 0x1;
> +			rnp->exp_seq_poll_rq = RCU_EXP_SEQ_POLL_DONE;
>  			INIT_WORK(&rnp->exp_poll_wq, sync_rcu_do_polled_gp);
>  		}
>  	}
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index c4a19c6a83cf..7ccb909d6355 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -910,14 +910,14 @@ static void sync_rcu_do_polled_gp(struct work_struct *wp)
>  	unsigned long s;
>  
>  	s = READ_ONCE(rnp->exp_seq_poll_rq);
> -	if (s & 0x1)
> +	if (s & RCU_EXP_SEQ_POLL_DONE)
>  		return;
>  	while (!sync_exp_work_done(s))
>  		__synchronize_rcu_expedited(true);

One additional question.  If we re-read rnp->exp_seq_poll_rq on each pass
through the loop, wouldn't we have less trouble with counter wrap?

							Thanx, Paul

>  	raw_spin_lock_irqsave(&rnp->exp_poll_lock, flags);
>  	s = rnp->exp_seq_poll_rq;
> -	if (!(s & 0x1) && sync_exp_work_done(s))
> -		WRITE_ONCE(rnp->exp_seq_poll_rq, s | 0x1);
> +	if (!(s & RCU_EXP_SEQ_POLL_DONE) && sync_exp_work_done(s))
> +		WRITE_ONCE(rnp->exp_seq_poll_rq, s | RCU_EXP_SEQ_POLL_DONE);
>  	raw_spin_unlock_irqrestore(&rnp->exp_poll_lock, flags);
>  }
>  
> @@ -946,7 +946,8 @@ unsigned long start_poll_synchronize_rcu_expedited(void)
>  	rnp = rdp->mynode;
>  	if (rcu_init_invoked())
>  		raw_spin_lock_irqsave(&rnp->exp_poll_lock, flags);
> -	if ((rnp->exp_seq_poll_rq & 0x1) || ULONG_CMP_LT(rnp->exp_seq_poll_rq, s)) {
> +	if ((rnp->exp_seq_poll_rq & RCU_EXP_SEQ_POLL_DONE) ||
> +	    ULONG_CMP_LT(rnp->exp_seq_poll_rq, s)) {
>  		WRITE_ONCE(rnp->exp_seq_poll_rq, s);
>  		if (rcu_init_invoked())
>  			queue_work(rcu_gp_wq, &rnp->exp_poll_wq);
> -- 
> 2.25.1
>