Re: [PATCH bpf v2] bpf: fix rcu lockdep warning for lpm_trie map_free callback

[David Miller <davem@davemloft.net>]

From: Yonghong Song <yhs@fb.com>
Date: Thu, 22 Feb 2018 10:10:35 -0800

> Commit 9a3efb6b661f ("bpf: fix memory leak in lpm_trie map_free callback function")
> fixed a memory leak and removed unnecessary locks in map_free callback function.
> Unfortrunately, it introduced a lockdep warning. When lockdep checking is turned on,
> running tools/testing/selftests/bpf/test_lpm_map will have:
> 
>   [   98.294321] =============================
>   [   98.294807] WARNING: suspicious RCU usage
>   [   98.295359] 4.16.0-rc2+ #193 Not tainted
>   [   98.295907] -----------------------------
>   [   98.296486] /home/yhs/work/bpf/kernel/bpf/lpm_trie.c:572 suspicious rcu_dereference_check() usage!
>   [   98.297657]
>   [   98.297657] other info that might help us debug this:
>   [   98.297657]
>   [   98.298663]
>   [   98.298663] rcu_scheduler_active = 2, debug_locks = 1
>   [   98.299536] 2 locks held by kworker/2:1/54:
>   [   98.300152]  #0:  ((wq_completion)"events"){+.+.}, at: [<00000000196bc1f0>] process_one_work+0x157/0x5c0
>   [   98.301381]  #1:  ((work_completion)(&map->work)){+.+.}, at: [<00000000196bc1f0>] process_one_work+0x157/0x5c0
> 
> Since actual trie tree removal happens only after no other
> accesses to the tree are possible, replacing
>   rcu_dereference_protected(*slot, lockdep_is_held(&trie->lock))
> with
>   rcu_dereference_protected(*slot, 1)
> fixed the issue.
> 
> Fixes: 9a3efb6b661f ("bpf: fix memory leak in lpm_trie map_free callback function")
> Reported-by: Eric Dumazet <edumazet@google.com>
> Suggested-by: Eric Dumazet <edumazet@google.com>
> Signed-off-by: Yonghong Song <yhs@fb.com>

Acked-by: David S. Miller <davem@davemloft.net>