Re: 4.19: Traced deadlock during xfrm_user module load

[Florian Westphal <fw@strlen.de>]

Thomas Jarosch <thomas.jarosch@intra2net.com> wrote:
> we're in the process of upgrading to kernel 4.19 and hit
> a very rare lockup on boot during "xfrm_user" module load.
> The tested kernel was 4.19.55.
> 
> When the strongswan IPsec service starts, it loads the xfrm_user module.
> -> modprobe hangs forever. 
> 
> Also network services like ssh or apache stop responding,
> ICMP ping still works.
> 
> By chance we had magic sysRq enabled and were able to get some meaningful stack 
> traces. We've rebuilt the kernel with LOCKDEP + DEBUG_INFO + DEBUG_INFO_REDUCED, 
> but so far failed to reproduce the issue even when hammering the suspected 
> deadlock case. Though it's just hammering it for a few hours yet.
> 
> Preliminary analysis:
> 
> "modprobe xfrm_user":
>     xfrm_user_init()
>         register_pernet_subsys()
>             -> grab pernet_ops_rwsem
>                 ..
>                 netlink_table_grab()
>                     calls schedule() as "nl_table_users" is non-zero
> 
> 
> conntrack netlink related program "info_iponline" does this in parallel:
>     netlink_bind()
>         netlink_lock_table() -> increases "nl_table_users"
>             nfnetlink_bind()
>             # does not unlock the table as it's locked by netlink_bind()
>                 __request_module()
>                     call_usermodehelper_exec()
>             
> 
> "modprobe nf_conntrack_netlink" runs and inits nf_conntrack_netlink:
>     ctnetlink_init()
>         register_pernet_subsys()
>             -> blocks on "pernet_ops_rwsem" thanks to xfrm_user module
>                 -> schedule()
>                     -> deadlock forever
> 

Thanks for this detailed analysis.
In this specific case I think this is enough:

diff --git a/net/netfilter/nfnetlink.c b/net/netfilter/nfnetlink.c
index 92077d459109..61ba92415480 100644
--- a/net/netfilter/nfnetlink.c
+++ b/net/netfilter/nfnetlink.c
@@ -578,7 +578,8 @@ static int nfnetlink_bind(struct net *net, int group)
        ss = nfnetlink_get_subsys(type << 8);
        rcu_read_unlock();
        if (!ss)
-               request_module("nfnetlink-subsys-%d", type);
+               request_module_nowait("nfnetlink-subsys-%d", type);
        return 0;
 }
 #endif