linux-kernel - [tip: core/rcu] rcu: Add WRITE_ONCE to rcu_node ->exp_seq

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Date:   Tue, 24 Mar 2020 09:16:54 -0000
From:   "tip-bot2 for Paul E. McKenney" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     "Paul E. McKenney" <paulmck@...nel.org>, x86 <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: [tip: core/rcu] rcu: Add WRITE_ONCE to rcu_node ->exp_seq_rq store

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     b0c18c87730a4de6da0303fa99aea43e814233f9
Gitweb:        https://git.kernel.org/tip/b0c18c87730a4de6da0303fa99aea43e814233f9
Author:        Paul E. McKenney <paulmck@...nel.org>
AuthorDate:    Fri, 03 Jan 2020 12:12:06 -08:00
Committer:     Paul E. McKenney <paulmck@...nel.org>
CommitterDate: Thu, 20 Feb 2020 15:58:22 -08:00

rcu: Add WRITE_ONCE to rcu_node ->exp_seq_rq store

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 <paulmck@...nel.org>
---
 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)
 				   sync_exp_work_done(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. */
 		spin_unlock(&rnp->exp_lock);
 		trace_rcu_exp_funnel_lock(rcu_state.name, rnp->level,
 					  rnp->grplo, rnp->grphi, TPS("nxtlvl"));