linux-kernel - Re: [PATCH] rcu_tree: avoid touch rnp->completed when starts a new GP

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Message-ID: <20150113093618.GI9719@linux.vnet.ibm.com>
Date:	Tue, 13 Jan 2015 01:36:18 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org,
	Josh Triplett <josh@...htriplett.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH] rcu_tree: avoid touch rnp->completed when starts a new GP

On Tue, Jan 13, 2015 at 03:30:34PM +0800, Lai Jiangshan wrote:
> In rcu_gp_init(), rnp->completed equals to rsp->completed in THEORY,
> we don't need to touch it normally.  If something goes wrong,
> it will complain and fixup rnp->completed and avoid oops.
> 
> Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>

Queued for 3.21, thank you!

							Thanx, Paul

> ---
>  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 7680fc2..ea6dc28 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1639,8 +1639,8 @@ static int rcu_gp_init(struct rcu_state *rsp)
>  		rcu_preempt_check_blocked_tasks(rnp);
>  		rnp->qsmask = rnp->qsmaskinit;
>  		ACCESS_ONCE(rnp->gpnum) = rsp->gpnum;
> -		WARN_ON_ONCE(rnp->completed != rsp->completed);
> -		ACCESS_ONCE(rnp->completed) = rsp->completed;
> +		if (WARN_ON_ONCE(rnp->completed != rsp->completed))
> +			ACCESS_ONCE(rnp->completed) = rsp->completed;
>  		if (rnp == rdp->mynode)
>  			(void)__note_gp_changes(rsp, rnp, rdp);
>  		rcu_preempt_boost_start_gp(rnp);
> -- 
> 2.1.0
> 

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