linux-kernel - Re: [PATCH tip/core/rcu 6/9] rcu: Upgrade sync_exp_work_done() to smp

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Date:   Thu, 6 Jun 2019 09:48:49 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        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 6/9] rcu: Upgrade sync_exp_work_done() to
 smp_mb()

On Thu, May 30, 2019 at 08:00:12AM -0700, Paul E. McKenney wrote:
> The sync_exp_work_done() function uses smp_mb__before_atomic(), but
> there is no obvious atomic in the ensuing code.  The ordering is
> absolutely required for grace periods to work correctly, so this
> commit upgrades the smp_mb__before_atomic() to smp_mb().
> 

Did this commit want a Fixes: line? Such that robots can find the right
kernels to backport this to?

> Reported-by: Andrea Parri <andrea.parri@...rulasolutions.com>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.ibm.com>
> ---
>  kernel/rcu/tree_exp.h | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index 9c990df880d1..d969650a72c6 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -259,8 +259,7 @@ static bool sync_exp_work_done(unsigned long s)
>  {
>  	if (rcu_exp_gp_seq_done(s)) {
>  		trace_rcu_exp_grace_period(rcu_state.name, s, TPS("done"));
> -		/* Ensure test happens before caller kfree(). */
> -		smp_mb__before_atomic(); /* ^^^ */
> +		smp_mb(); /* Ensure test happens before caller kfree(). */
>  		return true;
>  	}
>  	return false;
> -- 
> 2.17.1
>