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Subject[PATCH tip/core/rcu 14/30] rcu: Add READ_ONCE() to rcu_data ->gpwrap
Date
From: "Paul E. McKenney" <paulmck@kernel.org>

The rcu_data structure's ->gpwrap field is read locklessly, and so
this commit adds the required READ_ONCE() to a pair of laods in order
to avoid destructive compiler optimizations.

This data race was reported by KCSAN.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
kernel/rcu/tree.c | 2 +-
kernel/rcu/tree_stall.h | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index a70f56b..e851a12 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1322,7 +1322,7 @@ static void rcu_accelerate_cbs_unlocked(struct rcu_node *rnp,

rcu_lockdep_assert_cblist_protected(rdp);
c = rcu_seq_snap(&rcu_state.gp_seq);
- if (!rdp->gpwrap && ULONG_CMP_GE(rdp->gp_seq_needed, c)) {
+ if (!READ_ONCE(rdp->gpwrap) && ULONG_CMP_GE(rdp->gp_seq_needed, c)) {
/* Old request still live, so mark recent callbacks. */
(void)rcu_segcblist_accelerate(&rdp->cblist, c);
return;
diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index 3dcbecf..3275f27 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -602,7 +602,7 @@ void show_rcu_gp_kthreads(void)
continue;
for_each_leaf_node_possible_cpu(rnp, cpu) {
rdp = per_cpu_ptr(&rcu_data, cpu);
- if (rdp->gpwrap ||
+ if (READ_ONCE(rdp->gpwrap) ||
ULONG_CMP_GE(READ_ONCE(rcu_state.gp_seq),
READ_ONCE(rdp->gp_seq_needed)))
continue;
--
2.9.5
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