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Subject[PATCH tip/core/rcu 07/30] rcu: Add READ_ONCE() to rcu_node ->gp_seq
Date
From: "Paul E. McKenney" <paulmck@kernel.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@kernel.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
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