[PATCH 1/4] rcu/nocb: Protect lazy shrinker against concurrent (de-)offloading

Frederic Weisbecker
2023-03-22T19:45:47Z

The shrinker may run concurrently with callbacks (de-)offloading. As
such, calling rcu_nocb_lock() is very dangerous because it does a
conditional locking. The worst outcome is that rcu_nocb_lock() doesn't
lock but rcu_nocb_unlock() eventually unlocks, or the reverse, creating
an imbalance.

Fix this with protecting against (de-)offloading using the barrier mutex.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
kernel/rcu/tree_nocb.h | 17 ++++++++++++++++-
1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index f2280616f9d5..dd9b655ae533 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1336,13 +1336,25 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
unsigned long flags;
unsigned long count = 0;

+ /*
+ * Protect against concurrent (de-)offloading. Otherwise nocb locking
+ * may be ignored or imbalanced.
+ */
+ mutex_lock(&rcu_state.barrier_mutex);
+
/* Snapshot count of all CPUs */
for_each_possible_cpu(cpu) {
struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
- int _count = READ_ONCE(rdp->lazy_len);
+ int _count;
+
+ if (!rcu_rdp_is_offloaded(rdp))
+ continue;
+
+ _count = READ_ONCE(rdp->lazy_len);

if (_count == 0)
continue;
+
rcu_nocb_lock_irqsave(rdp, flags);
WRITE_ONCE(rdp->lazy_len, 0);
rcu_nocb_unlock_irqrestore(rdp, flags);
@@ -1352,6 +1364,9 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
if (sc->nr_to_scan <= 0)
break;
}
+
+ mutex_unlock(&rcu_state.barrier_mutex);
+
return count ? count : SHRINK_STOP;
}

--
2.34.1