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Date:   Fri, 14 Feb 2020 15:55:42 -0800
        "Paul E. McKenney" <>
Subject: [PATCH tip/core/rcu 05/30] rcu: Add WRITE_ONCE() to rcu_node ->qsmask update

From: "Paul E. McKenney" <>

The rcu_node structure's ->qsmask field is read locklessly, so this
commit adds the WRITE_ONCE() to an update in order to provide proper
documentation and READ_ONCE()/WRITE_ONCE() pairing.

This data race was reported by KCSAN.  Not appropriate for backporting
due to failure being unlikely.

Signed-off-by: Paul E. McKenney <>
 kernel/rcu/tree.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index d91c915..bb57c24 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1881,7 +1881,7 @@ static void rcu_report_qs_rnp(unsigned long mask, struct rcu_node *rnp,
 		WARN_ON_ONCE(oldmask); /* Any child must be all zeroed! */
 		WARN_ON_ONCE(!rcu_is_leaf_node(rnp) &&
-		rnp->qsmask &= ~mask;
+		WRITE_ONCE(rnp->qsmask, rnp->qsmask & ~mask);
 		trace_rcu_quiescent_state_report(, rnp->gp_seq,
 						 mask, rnp->qsmask, rnp->level,
 						 rnp->grplo, rnp->grphi,

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