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Date: Mon, 10 Feb 2020 04:29:10 -0800 From: Greg Kroah-Hartman <gregkh@...uxfoundation.org> To: linux-kernel@...r.kernel.org Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, stable@...r.kernel.org, syzbot+99f4ddade3c22ab0cf23@...kaller.appspotmail.com, "Paul E. McKenney" <paulmck@...nel.org>, Marco Elver <elver@...gle.com> Subject: [PATCH 5.5 037/367] rcu: Use READ_ONCE() for ->expmask in rcu_read_unlock_special() From: Paul E. McKenney <paulmck@...nel.org> commit c51f83c315c392d9776c33eb16a2fe1349d65c7f upstream. The rcu_node structure's ->expmask field is updated only when holding the ->lock, but is also accessed locklessly. This means that all ->expmask updates must use WRITE_ONCE() and all reads carried out without holding ->lock must use READ_ONCE(). This commit therefore changes the lockless ->expmask read in rcu_read_unlock_special() to use READ_ONCE(). Reported-by: syzbot+99f4ddade3c22ab0cf23@...kaller.appspotmail.com Signed-off-by: Paul E. McKenney <paulmck@...nel.org> Acked-by: Marco Elver <elver@...gle.com> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org> --- kernel/rcu/tree_plugin.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -612,7 +612,7 @@ static void rcu_read_unlock_special(stru t->rcu_read_unlock_special.b.exp_hint = false; exp = (t->rcu_blocked_node && t->rcu_blocked_node->exp_tasks) || - (rdp->grpmask & rnp->expmask) || + (rdp->grpmask & READ_ONCE(rnp->expmask)) || tick_nohz_full_cpu(rdp->cpu); // Need to defer quiescent state until everything is enabled. if (irqs_were_disabled && use_softirq &&
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