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Subject[PATCH v2 02/16] rcu/tree: Skip entry into the page allocator for PREEMPT_RT
Date
From: "Joel Fernandes (Google)" <joel@joelfernandes.org>

To keep the kfree_rcu() code working in purely atomic sections on RT,
such as non-threaded IRQ handlers and raw spinlock sections, avoid
calling into the page allocator which uses sleeping locks on RT.

In fact, even if the caller is preemptible, the kfree_rcu() code is
not, as the krcp->lock is a raw spinlock.

Calling into the page allocator is optional and avoiding it should be
Ok, especially with the page pre-allocation support in future patches.
Such pre-allocation would further avoid the a need for a dynamically
allocated page in the first place.

Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Uladzislau Rezki <urezki@gmail.com>
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>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
kernel/rcu/tree.c | 12 ++++++++++++
1 file changed, 12 insertions(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 6e967a9d6704..d28010cf158b 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3068,6 +3068,18 @@ 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);
}
--
2.20.1
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