linux-kernel - Re: [PATCH v3 4/4] rcu: Unlock non-start node only after accessing its gp_seq

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Message-Id: <20180521232537.GJ3803@linux.vnet.ibm.com>
Date:   Mon, 21 May 2018 16:25:38 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Joel Fernandes <joelaf@...gle.com>
Cc:     linux-kernel@...r.kernel.org,
        Joel Fernandes <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>, byungchul.park@....com,
        kernel-team@...roid.com
Subject: Re: [PATCH v3 4/4] rcu: Unlock non-start node only after accessing
 its gp_seq_needed

On Sun, May 20, 2018 at 09:42:20PM -0700, Joel Fernandes wrote:
> We acquire gp_seq_needed locklessly. To be safe, lets do the unlocking
> after the access.

Actually, no, we hold rnp_start's ->lock throughout.  And this CPU (or in
the case of no-CBs CPUs, this task) is in charge of rdp->gp_seq_needed,
so nothing else is accessing it.  Or at least that is the intent.  ;-)

One exception is CPU hotplug, but in that case, only the CPU doing the
hotplugging is allowed to touch rdp->gp_seq_needed and even then only
while the incoming/outgoing CPU is inactive.

							Thanx, Paul

> Signed-off-by: Joel Fernandes <joel@...lfernandes.org>
> ---
>  kernel/rcu/tree.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 879c67a31116..efbd21b2a1a6 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1603,13 +1603,13 @@ static bool rcu_start_this_gp(struct rcu_node *rnp_start, struct rcu_data *rdp,
>  	trace_rcu_grace_period(rsp->name, READ_ONCE(rsp->gp_seq), TPS("newreq"));
>  	ret = true;  /* Caller must wake GP kthread. */
>  unlock_out:
> -	if (rnp != rnp_start)
> -		raw_spin_unlock_rcu_node(rnp);
>  	/* Push furthest requested GP to leaf node and rcu_data structure. */
>  	if (ULONG_CMP_GE(rnp->gp_seq_needed, gp_seq_req)) {
>  		rnp_start->gp_seq_needed = gp_seq_req;
>  		rdp->gp_seq_needed = gp_seq_req;
>  	}
> +	if (rnp != rnp_start)
> +		raw_spin_unlock_rcu_node(rnp);
>  	return ret;
>  }
> 
> -- 
> 2.17.0.441.gb46fe60e1d-goog
>