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SubjectRe: [PATCH tip/core/rcu 1/4] srcu: Fix __call_srcu()/process_srcu() datarace
On Fri, Feb 14, 2020 at 04:29:29PM -0800, paulmck@kernel.org wrote:
> From: "Paul E. McKenney" <paulmck@kernel.org>
>
> The srcu_node structure's ->srcu_gp_seq_needed_exp field is accessed
> locklessly, so updates must use WRITE_ONCE(). This commit therefore
> adds the needed WRITE_ONCE() invocations.
>
> This data race was reported by KCSAN. Not appropriate for backporting
> due to failure being unlikely.

This does indeed look like there can be a failure; but this Changelog
fails to describe an actual problem.

> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> ---
> kernel/rcu/srcutree.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 657e6a7..b1edac9 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -550,7 +550,7 @@ static void srcu_gp_end(struct srcu_struct *ssp)
> snp->srcu_have_cbs[idx] = gpseq;
> rcu_seq_set_state(&snp->srcu_have_cbs[idx], 1);
> if (ULONG_CMP_LT(snp->srcu_gp_seq_needed_exp, gpseq))
> - snp->srcu_gp_seq_needed_exp = gpseq;
> + WRITE_ONCE(snp->srcu_gp_seq_needed_exp, gpseq);
> mask = snp->srcu_data_have_cbs[idx];
> snp->srcu_data_have_cbs[idx] = 0;
> spin_unlock_irq_rcu_node(snp);
> @@ -660,7 +660,7 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,
> if (snp == sdp->mynode)
> snp->srcu_data_have_cbs[idx] |= sdp->grpmask;
> if (!do_norm && ULONG_CMP_LT(snp->srcu_gp_seq_needed_exp, s))
> - snp->srcu_gp_seq_needed_exp = s;
> + WRITE_ONCE(snp->srcu_gp_seq_needed_exp, s);
> spin_unlock_irqrestore_rcu_node(snp, flags);
> }
>
> --
> 2.9.5
>

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