Fix a below race by not releasing a wait-head from the
GP-kthread as it can lead for reusing it whereas a worker
can still access it thus execute newly added callbacks too
early.
CPU 0 CPU 1
----- -----
// wait_tail == HEAD1
rcu_sr_normal_gp_cleanup() {
// has passed SR_MAX_USERS_WAKE_FROM_GP
wait_tail->next = next;
// done_tail = HEAD1
smp_store_release(&rcu_state.srs_done_tail, wait_tail);
queue_work() {
test_and_set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(work)
__queue_work()
}
}
set_work_pool_and_clear_pending()
rcu_sr_normal_gp_cleanup_work() {
// new GP, wait_tail == HEAD2
rcu_sr_normal_gp_cleanup() {
// executes all completion, but stop at HEAD1
wait_tail->next = HEAD1;
// done_tail = HEAD2
smp_store_release(&rcu_state.srs_done_tail, wait_tail);
queue_work() {
test_and_set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(work)
__queue_work()
}
}
// done = HEAD2
done = smp_load_acquire(&rcu_state.srs_done_tail);
// head = HEAD1
head = done->next;
done->next = NULL;
llist_for_each_safe() {
// completes all callbacks, release HEAD1
}
}
// Process second queue
set_work_pool_and_clear_pending()
rcu_sr_normal_gp_cleanup_work() {
// done = HEAD2
done = smp_load_acquire(&rcu_state.srs_done_tail);
// new GP, wait_tail == HEAD3
rcu_sr_normal_gp_cleanup() {
// Finds HEAD2 with ->next == NULL at the end
rcu_sr_put_wait_head(HEAD2)
...
// A few more GPs later
rcu_sr_normal_gp_init() {
HEAD2 = rcu_sr_get_wait_head();
llist_add(HEAD2, &rcu_state.srs_next);
// head == rcu_state.srs_next
head = done->next;
done->next = NULL;
llist_for_each_safe() {
// EXECUTE CALLBACKS TOO EARLY!!!
}
}
Reported-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Fixes: 05a10b921000 ("rcu: Support direct wake-up of synchronize_rcu() users")
Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
---
kernel/rcu/tree.c | 22 ++++++++--------------
1 file changed, 8 insertions(+), 14 deletions(-)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 31f3a61f9c38..475647620b12 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1656,21 +1656,11 @@ static void rcu_sr_normal_gp_cleanup(void)
WARN_ON_ONCE(!rcu_sr_is_wait_head(wait_tail));
/*
- * Process (a) and (d) cases. See an illustration. Apart of
- * that it handles the scenario when all clients are done,
- * wait-head is released if last. The worker is not kicked.
+ * Process (a) and (d) cases. See an illustration.
*/
llist_for_each_safe(rcu, next, wait_tail->next) {
- if (rcu_sr_is_wait_head(rcu)) {
- if (!rcu->next) {
- rcu_sr_put_wait_head(rcu);
- wait_tail->next = NULL;
- } else {
- wait_tail->next = rcu;
- }
-
+ if (rcu_sr_is_wait_head(rcu))
break;
- }
rcu_sr_normal_complete(rcu);
// It can be last, update a next on this step.
@@ -1684,8 +1674,12 @@ static void rcu_sr_normal_gp_cleanup(void)
smp_store_release(&rcu_state.srs_done_tail, wait_tail);
ASSERT_EXCLUSIVE_WRITER(rcu_state.srs_done_tail);
- if (wait_tail->next)
- queue_work(system_highpri_wq, &rcu_state.srs_cleanup_work);
+ /*
+ * We schedule a work in order to perform a final processing
+ * of outstanding users(if still left) and releasing wait-heads
+ * added by rcu_sr_normal_gp_init() call.
+ */
+ queue_work(system_highpri_wq, &rcu_state.srs_cleanup_work);
}
/*
--
2.39.2
