On Sat, 9 May 2020 12:52:44 +0530 Amol Grover wrote:
> ipmr_for_each_table() uses list_for_each_entry_rcu() for
> traversing outside of an RCU read-side critical section but
> under the protection of pernet_ops_rwsem. Hence add the
> corresponding lockdep expression to silence the following
> false-positive warning at boot:
Thanks for the fix, the warning has been annoying me as well!
> [ 0.645292] =============================
> [ 0.645294] WARNING: suspicious RCU usage
> [ 0.645296] 5.5.4-stable #17 Not tainted
> [ 0.645297] -----------------------------
> [ 0.645299] net/ipv4/ipmr.c:136 RCU-list traversed in non-reader section!!
please provide a fuller stack trace, it would have helped the review
> Signed-off-by: Amol Grover <frextr...@gmail.com>
> ---
> net/ipv4/ipmr.c | 7 ++++---
> 1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/net/ipv4/ipmr.c b/net/ipv4/ipmr.c
> index 99c864eb6e34..950ffe9943da 100644
> --- a/net/ipv4/ipmr.c
> +++ b/net/ipv4/ipmr.c
> @@ -109,9 +109,10 @@ static void mroute_clean_tables(struct mr_table *mrt,
> int flags);
> static void ipmr_expire_process(struct timer_list *t);
>
> #ifdef CONFIG_IP_MROUTE_MULTIPLE_TABLES
> -#define ipmr_for_each_table(mrt, net) \
> - list_for_each_entry_rcu(mrt, &net->ipv4.mr_tables, list, \
> - lockdep_rtnl_is_held())
> +#define ipmr_for_each_table(mrt, net)
> \
> + list_for_each_entry_rcu(mrt, &net->ipv4.mr_tables, list, \
> + lockdep_rtnl_is_held() || \
> + lockdep_is_held(&pernet_ops_rwsem))
This is a strange condition, IMHO. How can we be fine with either
lock.. This is supposed to be the writer side lock, one can't have
two writer side locks..
I think what is happening is this:
ipmr_net_init() -> ipmr_rules_init() -> ipmr_new_table()
ipmr_new_table() returns an existing table if there is one, but
obviously none can exist at init. So a better fix would be:
#define ipmr_for_each_table(mrt, net) \
list_for_each_entry_rcu(mrt, &net->ipv4.mr_tables, list, \
lockdep_rtnl_is_held() || \
list_empty(&net->ipv4.mr_tables))
Thoughts?
