linux-kernel - [PATCH tip/core/rcu 07/30] rcu: Add READ_ONCE() to rcu_node ->gp

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Message-Id: <20200214235607.13749-7-paulmck@kernel.org>
Date:   Fri, 14 Feb 2020 15:55:44 -0800
From:   paulmck@...nel.org
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: [PATCH tip/core/rcu 07/30] rcu: Add READ_ONCE() to rcu_node ->gp_seq

From: "Paul E. McKenney" <paulmck@...nel.org>

The rcu_node structure's ->gp_seq field is read locklessly, so this
commit adds the READ_ONCE() to several loads in order to avoid
destructive compiler optimizations.

This data race was reported by KCSAN.  Not appropriate for backporting
because this affects only tracing and warnings.

Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/tree.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index bb57c24..236692d 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1126,8 +1126,9 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
 static void trace_rcu_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
 			      unsigned long gp_seq_req, const char *s)
 {
-	trace_rcu_future_grace_period(rcu_state.name, rnp->gp_seq, gp_seq_req,
-				      rnp->level, rnp->grplo, rnp->grphi, s);
+	trace_rcu_future_grace_period(rcu_state.name, READ_ONCE(rnp->gp_seq),
+				      gp_seq_req, rnp->level,
+				      rnp->grplo, rnp->grphi, s);
 }
 
 /*
@@ -1904,7 +1905,7 @@ static void rcu_report_qs_rnp(unsigned long mask, struct rcu_node *rnp,
 		rnp_c = rnp;
 		rnp = rnp->parent;
 		raw_spin_lock_irqsave_rcu_node(rnp, flags);
-		oldmask = rnp_c->qsmask;
+		oldmask = READ_ONCE(rnp_c->qsmask);
 	}
 
 	/*
@@ -2052,7 +2053,7 @@ int rcutree_dying_cpu(unsigned int cpu)
 		return 0;
 
 	blkd = !!(rnp->qsmask & rdp->grpmask);
-	trace_rcu_grace_period(rcu_state.name, rnp->gp_seq,
+	trace_rcu_grace_period(rcu_state.name, READ_ONCE(rnp->gp_seq),
 			       blkd ? TPS("cpuofl") : TPS("cpuofl-bgp"));
 	return 0;
 }
-- 
2.9.5