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Date:   Fri, 14 Feb 2020 15:55:43 -0800
        "Paul E. McKenney" <>
Subject: [PATCH tip/core/rcu 06/30] rcu: Add WRITE_ONCE to rcu_node ->exp_seq_rq store

From: "Paul E. McKenney" <>

The rcu_node structure's ->exp_seq_rq field is read locklessly, so
this commit adds the WRITE_ONCE() to a load 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_exp.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index d7e0484..85b009e 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -314,7 +314,7 @@ static bool exp_funnel_lock(unsigned long s)
 			return true;
-		rnp->exp_seq_rq = s; /* Followers can wait on us. */
+		WRITE_ONCE(rnp->exp_seq_rq, s); /* Followers can wait on us. */
 		trace_rcu_exp_funnel_lock(, rnp->level,
 					  rnp->grplo, rnp->grphi, TPS("nxtlvl"));

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