linux-kernel - [PATCH 4.14 3/9] bpf: fix rcu lockdep warning for lpm_trie map

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Date:   Fri,  9 Mar 2018 16:19:08 -0800
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        "ast@...nel.org, daniel@...earbox.net, stable@...r.kernel.org, Yonghong
        Song" <yhs@...com>, Eric Dumazet <edumazet@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Yonghong Song <yhs@...com>
Subject: [PATCH 4.14 3/9] bpf: fix rcu lockdep warning for lpm_trie map_free callback

4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Yonghong Song <yhs@...com>

[ upstream commit 6c5f61023c5b0edb0c8a64c902fe97c6453b1852 ]

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@...gle.com>
Suggested-by: Eric Dumazet <edumazet@...gle.com>
Signed-off-by: Yonghong Song <yhs@...com>
Reviewed-by: Eric Dumazet <edumazet@...gle.com>
Acked-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 kernel/bpf/lpm_trie.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

--- a/kernel/bpf/lpm_trie.c
+++ b/kernel/bpf/lpm_trie.c
@@ -484,8 +484,7 @@ static void trie_free(struct bpf_map *ma
 		slot = &trie->root;
 
 		for (;;) {
-			node = rcu_dereference_protected(*slot,
-					lockdep_is_held(&trie->lock));
+			node = rcu_dereference_protected(*slot, 1);
 			if (!node)
 				goto out;