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SubjectRe: [PATCH tip/core/rcu 4/4] srcu: Add READ_ONCE() to srcu_struct ->srcu_gp_seq load
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) {

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