linux-kernel - Re: [PATCH tip/core/rcu 4/4] srcu: Add READ_ONCE() to srcu_struct ->srcu_gp

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Date:   Mon, 17 Feb 2020 13:45:07 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     paulmck@...nel.org
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, mingo@...nel.org, jiangshanlai@...il.com,
        dipankar@...ibm.com, akpm@...ux-foundation.org,
        mathieu.desnoyers@...icios.com, josh@...htriplett.org,
        tglx@...utronix.de, rostedt@...dmis.org, dhowells@...hat.com,
        edumazet@...gle.com, fweisbec@...il.com, oleg@...hat.com,
        joel@...lfernandes.org
Subject: Re: [PATCH tip/core/rcu 4/4] srcu: Add READ_ONCE() to srcu_struct
 ->srcu_gp_seq load

On Fri, Feb 14, 2020 at 04:29:32PM -0800, paulmck@...nel.org wrote:
> From: "Paul E. McKenney" <paulmck@...nel.org>
> 
> The load of the srcu_struct structure's ->srcu_gp_seq field in
> srcu_funnel_gp_start() is lockless, so this commit adds the requisite
> READ_ONCE().
> 
> This data race was reported by KCSAN.

But is there in actual fact a data-race? AFAICT this code was just fine.

> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> ---
>  kernel/rcu/srcutree.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 119a373..90ab475 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -678,7 +678,7 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,
>  
>  	/* If grace period not already done and none in progress, start it. */
>  	if (!rcu_seq_done(&ssp->srcu_gp_seq, s) &&
> -	    rcu_seq_state(ssp->srcu_gp_seq) == SRCU_STATE_IDLE) {
> +	    rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq)) == SRCU_STATE_IDLE) {
>  		WARN_ON_ONCE(ULONG_CMP_GE(ssp->srcu_gp_seq, ssp->srcu_gp_seq_needed));
>  		srcu_gp_start(ssp);
>  		if (likely(srcu_init_done))
> -- 
> 2.9.5
>