diff options
| author | Cong Wang <xiyou.wangcong@gmail.com> | 2017-09-11 16:33:32 -0700 |
|---|---|---|
| committer | David S. Miller <davem@davemloft.net> | 2017-09-12 20:41:02 -0700 |
| commit | 1697c4bb5245649a23f06a144cc38c06715e1b65 (patch) | |
| tree | 020f13f2d700eeb85998258429a05d119d689937 /net/sched | |
| parent | e2ef75445340ca7ec2c4558f84ae6c8c5d650fc8 (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.c | 24 |
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); |