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Subject[PATCH tip/core/rcu 1/3] rcu-tasks: *_ONCE() for rcu_tasks_cbs_head
Date
From: "Paul E. McKenney" <paulmck@kernel.org>

The RCU tasks list of callbacks, rcu_tasks_cbs_head, is sampled locklessly
by rcu_tasks_kthread() when waiting for work to do. This commit therefore
applies READ_ONCE() to that lockless sampling and WRITE_ONCE() to the
single potential store outside of rcu_tasks_kthread.

This data race was reported by KCSAN. Not appropriate for backporting
due to failure being unlikely.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
kernel/rcu/update.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 6c4b862..a27df76 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -528,7 +528,7 @@ void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func)
rhp->func = func;
raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags);
needwake = !rcu_tasks_cbs_head;
- *rcu_tasks_cbs_tail = rhp;
+ WRITE_ONCE(*rcu_tasks_cbs_tail, rhp);
rcu_tasks_cbs_tail = &rhp->next;
raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags);
/* We can't create the thread unless interrupts are enabled. */
@@ -658,7 +658,7 @@ static int __noreturn rcu_tasks_kthread(void *arg)
/* If there were none, wait a bit and start over. */
if (!list) {
wait_event_interruptible(rcu_tasks_cbs_wq,
- rcu_tasks_cbs_head);
+ READ_ONCE(rcu_tasks_cbs_head));
if (!rcu_tasks_cbs_head) {
WARN_ON(signal_pending(current));
schedule_timeout_interruptible(HZ/10);
--
2.9.5
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