linux-kernel - Re: [PATCH tip/core/rcu 06/30] rcu: Add WRITE_ONCE to rcu_node ->exp_seq

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Date:   Fri, 14 Feb 2020 22:47:43 -0500
From:   Steven Rostedt <rostedt@...dmis.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, peterz@...radead.org, dhowells@...hat.com,
        edumazet@...gle.com, fweisbec@...il.com, oleg@...hat.com,
        joel@...lfernandes.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH tip/core/rcu 06/30] rcu: Add WRITE_ONCE to rcu_node
 ->exp_seq_rq store

On Fri, 14 Feb 2020 15:55:43 -0800
paulmck@...nel.org wrote:

> From: "Paul E. McKenney" <paulmck@...nel.org>
> 
> The rcu_node structure's ->exp_seq_rq field is read locklessly, so
> this commit adds the WRITE_ONCE() to a load in order to provide proper
> documentation and READ_ONCE()/WRITE_ONCE() pairing.
> 
> This data race was reported by KCSAN.  Not appropriate for backporting
> due to failure being unlikely.
> 
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> ---
>  kernel/rcu/tree_exp.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index d7e0484..85b009e 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -314,7 +314,7 @@ static bool exp_funnel_lock(unsigned long s)
>  				   sync_exp_work_done(s));
>  			return true;
>  		}
> -		rnp->exp_seq_rq = s; /* Followers can wait on us. */
> +		WRITE_ONCE(rnp->exp_seq_rq, s); /* Followers can wait on us. */

Didn't Linus say this is basically bogus?

Perhaps just using it as documenting that it's read locklessly, but is
it really needed?

-- Steve



>  		spin_unlock(&rnp->exp_lock);
>  		trace_rcu_exp_funnel_lock(rcu_state.name, rnp->level,
>  					  rnp->grplo, rnp->grphi, TPS("nxtlvl"));