linux-kernel - Re: [PATCH RFC 5/5] rcuwait: Replace rcu_assign_pointer() with WRITE

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Message-ID: <20190221092020.GZ32477@hirez.programming.kicks-ass.net>
Date:   Thu, 21 Feb 2019 10:20:20 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Christian Brauner <christian@...uner.io>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Ahern <dsahern@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Ido Schimmel <idosch@...lanox.com>,
        Ingo Molnar <mingo@...hat.com>,
        "moderated list:INTEL ETHERNET DRIVERS" 
        <intel-wired-lan@...ts.osuosl.org>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Josh Triplett <josh@...htriplett.org>, keescook@...omium.org,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Martin KaFai Lau <kafai@...com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        netdev@...r.kernel.org, "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        rcu@...r.kernel.org, Song Liu <songliubraving@...com>,
        Steven Rostedt <rostedt@...dmis.org>,
        xdp-newbies@...r.kernel.org, Yonghong Song <yhs@...com>
Subject: Re: [PATCH RFC 5/5] rcuwait: Replace rcu_assign_pointer() with
 WRITE_ONCE

On Thu, Feb 21, 2019 at 12:49:42AM -0500, Joel Fernandes (Google) wrote:
> This suppresses a sparse error generated due to the recently added
> rcu_assign_pointer sparse check below. It seems WRITE_ONCE should be
> sufficient here.
> 
> >> kernel//locking/percpu-rwsem.c:162:9: sparse: error: incompatible
> types in comparison expression (different address spaces)
> 
> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> ---
>  include/linux/rcuwait.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/rcuwait.h b/include/linux/rcuwait.h
> index 90bfa3279a01..9e5b4760e6c2 100644
> --- a/include/linux/rcuwait.h
> +++ b/include/linux/rcuwait.h
> @@ -44,7 +44,7 @@ extern void rcuwait_wake_up(struct rcuwait *w);
>  	 */                                                             \
>  	WARN_ON(current->exit_state);                                   \
>  									\
> -	rcu_assign_pointer((w)->task, current);				\
> +	WRITE_ONCE((w)->task, current);					\
>  	for (;;) {							\
>  		/*							\
>  		 * Implicit barrier (A) pairs with (B) in		\

Distinct lack of justification for loosing the RELEASE again.