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Date:   Tue, 18 Feb 2020 12:46:31 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com,
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        tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com,
        edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com,
        joel@joelfernandes.org
Subject: Re: [PATCH tip/core/rcu 4/4] srcu: Add READ_ONCE() to srcu_struct
 ->srcu_gp_seq load
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On Mon, Feb 17, 2020 at 03:06:57PM -0800, Paul E. McKenney wrote:
> commit 52324a7b8a025f47a1a1a9fbd23ffe59fa764764
> Author: Paul E. McKenney <paulmck@kernel.org>
> Date:   Fri Jan 3 11:42:05 2020 -0800
> 
>     srcu: Hold srcu_struct ->lock when updating ->srcu_gp_seq
>     
>     A read of the srcu_struct structure's ->srcu_gp_seq field should not
>     need READ_ONCE() when that structure's ->lock is held.  Except that this
>     lock is not always held when updating this field.  This commit therefore
>     acquires the lock around updates and removes a now-unneeded READ_ONCE().
>     
>     This data race was reported by KCSAN.
>     
>     Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

> 
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 119a373..c19c1df 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -450,7 +450,7 @@ static void srcu_gp_start(struct srcu_struct *ssp)
>  	spin_unlock_rcu_node(sdp);  /* Interrupts remain disabled. */
>  	smp_mb(); /* Order prior store to ->srcu_gp_seq_needed vs. GP start. */
>  	rcu_seq_start(&ssp->srcu_gp_seq);
> -	state = rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq));
> +	state = rcu_seq_state(ssp->srcu_gp_seq);
>  	WARN_ON_ONCE(state != SRCU_STATE_SCAN1);
>  }
>  
> @@ -1130,7 +1130,9 @@ static void srcu_advance_state(struct srcu_struct *ssp)
>  			return; /* readers present, retry later. */
>  		}
>  		srcu_flip(ssp);
> +		spin_lock_irq_rcu_node(ssp);
>  		rcu_seq_set_state(&ssp->srcu_gp_seq, SRCU_STATE_SCAN2);
> +		spin_unlock_irq_rcu_node(ssp);
>  	}
>  
>  	if (rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq)) == SRCU_STATE_SCAN2) {