[PATCH net] net: sched: flower: don't call synchronize_rcu() on mask creation

Vlad Buslov
2019-06-13T14:59:35Z

Current flower mask creating code assumes that temporary mask that is used
when inserting new filter is stack allocated. To prevent race condition
with data patch synchronize_rcu() is called every time fl_create_new_mask()
replaces temporary stack allocated mask. As reported by Jiri, this
increases runtime of creating 20000 flower classifiers from 4 seconds to
163 seconds. However, this design is no longer necessary since temporary
mask was converted to be dynamically allocated by commit 2cddd2014782
("net/sched: cls_flower: allocate mask dynamically in fl_change()").

Remove synchronize_rcu() calls from mask creation code. Instead, refactor
fl_change() to always deallocate temporary mask with rcu grace period.

Fixes: 195c234d15c9 ("net: sched: flower: handle concurrent mask insertion")
Reported-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Vlad Buslov <vladbu@mellanox.com>
---
net/sched/cls_flower.c | 34 ++++++++++++++++++----------------
1 file changed, 18 insertions(+), 16 deletions(-)

diff --git a/net/sched/cls_flower.c b/net/sched/cls_flower.c
index c388372df0e2..eedd5786c084 100644
--- a/net/sched/cls_flower.c
+++ b/net/sched/cls_flower.c
@@ -320,10 +320,13 @@ static int fl_init(struct tcf_proto *tp)
return rhashtable_init(&head->ht, &mask_ht_params);
}

-static void fl_mask_free(struct fl_flow_mask *mask)
+static void fl_mask_free(struct fl_flow_mask *mask, bool mask_init_done)
{
- WARN_ON(!list_empty(&mask->filters));
- rhashtable_destroy(&mask->ht);
+ /* temporary masks don't have their filters list and ht initialized */
+ if (mask_init_done) {
+ WARN_ON(!list_empty(&mask->filters));
+ rhashtable_destroy(&mask->ht);
+ }
kfree(mask);
}

@@ -332,7 +335,15 @@ static void fl_mask_free_work(struct work_struct *work)
struct fl_flow_mask *mask = container_of(to_rcu_work(work),
struct fl_flow_mask, rwork);

- fl_mask_free(mask);
+ fl_mask_free(mask, true);
+}
+
+static void fl_uninit_mask_free_work(struct work_struct *work)
+{
+ struct fl_flow_mask *mask = container_of(to_rcu_work(work),
+ struct fl_flow_mask, rwork);
+
+ fl_mask_free(mask, false);
}

static bool fl_mask_put(struct cls_fl_head *head, struct fl_flow_mask *mask)
@@ -1346,9 +1357,6 @@ static struct fl_flow_mask *fl_create_new_mask(struct cls_fl_head *head,
if (err)
goto errout_destroy;

- /* Wait until any potential concurrent users of mask are finished */
- synchronize_rcu();
-
spin_lock(&head->masks_lock);
list_add_tail_rcu(&newmask->list, &head->masks);
spin_unlock(&head->masks_lock);
@@ -1375,11 +1383,7 @@ static int fl_check_assign_mask(struct cls_fl_head *head,

/* Insert mask as temporary node to prevent concurrent creation of mask
* with same key. Any concurrent lookups with same key will return
- * -EAGAIN because mask's refcnt is zero. It is safe to insert
- * stack-allocated 'mask' to masks hash table because we call
- * synchronize_rcu() before returning from this function (either in case
- * of error or after replacing it with heap-allocated mask in
- * fl_create_new_mask()).
+ * -EAGAIN because mask's refcnt is zero.
*/
fnew->mask = rhashtable_lookup_get_insert_fast(&head->ht,
&mask->ht_node,
@@ -1414,8 +1418,6 @@ static int fl_check_assign_mask(struct cls_fl_head *head,
errout_cleanup:
rhashtable_remove_fast(&head->ht, &mask->ht_node,
mask_ht_params);
- /* Wait until any potential concurrent users of mask are finished */
- synchronize_rcu();
return ret;
}

@@ -1644,7 +1646,7 @@ static int fl_change(struct net *net, struct sk_buff *in_skb,
*arg = fnew;

kfree(tb);
- kfree(mask);
+ tcf_queue_work(&mask->rwork, fl_uninit_mask_free_work);
return 0;

errout_ht:
@@ -1664,7 +1666,7 @@ static int fl_change(struct net *net, struct sk_buff *in_skb,
errout_tb:
kfree(tb);
errout_mask_alloc:
- kfree(mask);
+ tcf_queue_work(&mask->rwork, fl_uninit_mask_free_work);
errout_fold:
if (fold)
__fl_put(fold);
--
2.21.0