[PATCH rcu/dev -fixes 2/4] rcu/tree: Skip entry into the page allocator for PREEMPT_RT

Joel Fernandes (Google)
2020-04-20T15:39:04Z

To keep kfree_rcu() path working on raw non-preemptible sections,
prevent the optional entry into the allocator as it uses sleeping locks.
In fact, even if the caller of kfree_rcu() is preemptible, this path
still is not, as krcp->lock is a raw spinlock as done in previous
patches. With additional page pre-allocation in the works, hitting this
return is going to be much less likely soon so just prevent it for now
so that PREEMPT_RT does not break. Note that page allocation here is an
optimization and skipping it still makes kfree_rcu() work.

Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Co-developed-by: Uladzislau Rezki <urezki@gmail.com>
Signed-off-by: Uladzislau Rezki <urezki@gmail.com>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
kernel/rcu/tree.c | 12 ++++++++++++
1 file changed, 12 insertions(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index cf68d3d9f5b81..cd61649e1b001 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3092,6 +3092,18 @@ kfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp,
if (!bnode) {
WARN_ON_ONCE(sizeof(struct kfree_rcu_bulk_data) > PAGE_SIZE);

+ /*
+ * To keep this path working on raw non-preemptible
+ * sections, prevent the optional entry into the
+ * allocator as it uses sleeping locks. In fact, even
+ * if the caller of kfree_rcu() is preemptible, this
+ * path still is not, as krcp->lock is a raw spinlock.
+ * With additional page pre-allocation in the works,
+ * hitting this return is going to be much less likely.
+ */
+ if (IS_ENABLED(CONFIG_PREEMPT_RT))
+ return false;
+
bnode = (struct kfree_rcu_bulk_data *)
__get_free_page(GFP_NOWAIT | __GFP_NOWARN);
}
--
2.26.1.301.g55bc3eb7cb9-goog