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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>
This commit is contained in:
Cong Wang 2017-09-11 16:33:32 -07:00 committed by David S. Miller
parent e2ef754453
commit 1697c4bb52
1 changed files with 19 additions and 7 deletions
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26
net/sched/cls_api.c
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@ -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) {
tcf_chain_flush(chain);
rcu_barrier();
}
/* 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);
/* 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);