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authorCong Wang <xiyou.wangcong@gmail.com>2017-09-11 16:33:32 -0700
committerDavid S. Miller <davem@davemloft.net>2017-09-12 20:41:02 -0700
commit1697c4bb5245649a23f06a144cc38c06715e1b65 (patch)
tree020f13f2d700eeb85998258429a05d119d689937 /net/sched
parente2ef75445340ca7ec2c4558f84ae6c8c5d650fc8 (diff)
net_sched: carefully handle tcf_block_put()
As pointed out by Jiri, there is still a race condition between tcf_block_put() and tcf_chain_destroy() in a RCU callback. There is no way to make it correct without proper locking or synchronization, because both operate on a shared list. Locking is hard, because the only lock we can pick here is a spinlock, however, in tc_dump_tfilter() we iterate this list with a sleeping function called (tcf_chain_dump()), which makes using a lock to protect chain_list almost impossible. Jiri suggested the idea of holding a refcnt before flushing, this works because it guarantees us there would be no parallel tcf_chain_destroy() during the loop, therefore the race condition is gone. But we have to be very careful with proper synchronization with RCU callbacks. Suggested-by: Jiri Pirko <jiri@mellanox.com> Cc: Jamal Hadi Salim <jhs@mojatatu.com> Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com> Acked-by: Jiri Pirko <jiri@mellanox.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'net/sched')
-rw-r--r--net/sched/cls_api.c24
1 files changed, 18 insertions, 6 deletions
diff --git a/net/sched/cls_api.c b/net/sched/cls_api.c
index d29e79d98a69..0b2219adf520 100644
--- a/net/sched/cls_api.c
+++ b/net/sched/cls_api.c
@@ -275,15 +275,27 @@ void tcf_block_put(struct tcf_block *block)
/* XXX: Standalone actions are not allowed to jump to any chain, and
* bound actions should be all removed after flushing. However,
- * filters are destroyed in RCU callbacks, we have to flush and wait
- * for them inside the loop, otherwise we race with RCU callbacks on
- * this list.
+ * filters are destroyed in RCU callbacks, we have to hold the chains
+ * first, otherwise we would always race with RCU callbacks on this list
+ * without proper locking.
*/
- list_for_each_entry_safe(chain, tmp, &block->chain_list, list) {
+
+ /* Wait for existing RCU callbacks to cool down. */
+ rcu_barrier();
+
+ /* Hold a refcnt for all chains, except 0, in case they are gone. */
+ list_for_each_entry(chain, &block->chain_list, list)
+ if (chain->index)
+ tcf_chain_hold(chain);
+
+ /* No race on the list, because no chain could be destroyed. */
+ list_for_each_entry(chain, &block->chain_list, list)
tcf_chain_flush(chain);
- rcu_barrier();
- }
+ /* Wait for RCU callbacks to release the reference count. */
+ rcu_barrier();
+
+ /* At this point, all the chains should have refcnt == 1. */
list_for_each_entry_safe(chain, tmp, &block->chain_list, list)
tcf_chain_put(chain);
kfree(block);