[PATCH 20/24] rcu/tree: Make kvfree_rcu() tolerate any alignment

Uladzislau Rezki (Sony)
2020-04-28T21:00:15Z

From: "Joel Fernandes (Google)" <joel@joelfernandes.org>

Handle cases where the the object being kvfree_rcu()'d is not aligned by
2-byte boundaries.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
kernel/rcu/tree.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 501cac02146d..649bad7ad0f0 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2877,6 +2877,9 @@ struct kvfree_rcu_bulk_data {
#define KVFREE_BULK_MAX_ENTR \
((PAGE_SIZE - sizeof(struct kvfree_rcu_bulk_data)) / sizeof(void *))

+/* Encoding the offset of a fake rcu_head to indicate the head is a wrapper. */
+#define RCU_HEADLESS_KFREE BIT(31)
+
/**
* struct kfree_rcu_cpu_work - single batch of kfree_rcu() requests
* @rcu_work: Let queue_rcu_work() invoke workqueue handler after grace period
@@ -3078,9 +3081,9 @@ static void kfree_rcu_work(struct work_struct *work)
next = head->next;

/* We tag the headless object, if so adjust offset. */
- headless = (((unsigned long) head - offset) & BIT(0));
+ headless = !!(offset & RCU_HEADLESS_KFREE);
if (headless)
- offset -= 1;
+ offset &= ~(RCU_HEADLESS_KFREE);

ptr = (void *) head - offset;

@@ -3356,7 +3359,7 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
* that has to be freed as well as dynamically
* attached wrapper/head.
*/
- func = (rcu_callback_t) (sizeof(unsigned long *) + 1);
+ func = (rcu_callback_t)(sizeof(unsigned long *) | RCU_HEADLESS_KFREE);
}

head->func = func;
--
2.20.1