From: Martin KaFai Lau <martin.lau@linux.dev>
To: David Carlier <devnexen@gmail.com>
Cc: Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
davem@davemloft.net, Jakub Kicinski <kuba@kernel.org>,
netdev@vger.kernel.org, bpf <bpf@vger.kernel.org>
Subject: Re: [PATCH bpf-next] bpf: use RCU-safe iteration in dev_map_redirect_multi() SKB path
Date: Tue, 24 Mar 2026 16:00:09 -0700 [thread overview]
Message-ID: <9985d6b0-d305-429f-9dac-a9a40085c843@linux.dev> (raw)
In-Reply-To: <20260320072645.16731-1-devnexen@gmail.com>
On 3/20/26 12:26 AM, David Carlier wrote:
> The DEVMAP_HASH branch in dev_map_redirect_multi() uses
> hlist_for_each_entry_safe() to iterate hash buckets, but this function
> runs under RCU protection (called from xdp_do_generic_redirect_map()
> in softirq context). Concurrent writers (__dev_map_hash_update_elem,
> dev_map_hash_delete_elem) modify the list using RCU primitives
> (hlist_add_head_rcu, hlist_del_rcu).
>
> hlist_for_each_entry_safe() performs plain pointer dereferences without
> rcu_dereference(), missing the acquire barrier needed to pair with
> writers' rcu_assign_pointer(). On weakly-ordered architectures (ARM64,
> POWER), a reader can observe a partially-constructed node. It also
> defeats CONFIG_PROVE_RCU lockdep validation and KCSAN data-race
> detection.
>
> Replace with hlist_for_each_entry_rcu() using rcu_read_lock_bh_held()
> as the lockdep condition, consistent with the rcu_dereference_check()
> used in the DEVMAP (non-hash) branch of the same functions. Also fix
> the same incorrect lockdep_is_held(&dtab->index_lock) condition in
> dev_map_enqueue_multi(), where the lock is not held either.
Applied. Thanks.
Take a look at the review here:
https://sashiko.dev/#/patchset/20260320072645.16731-1-devnexen%40gmail.com
The first comment is legit and please followup. The second comment looks
unrelated.
prev parent reply other threads:[~2026-03-24 23:00 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-03-20 7:26 [PATCH bpf-next] bpf: use RCU-safe iteration in dev_map_redirect_multi() SKB path David Carlier
2026-03-24 23:00 ` patchwork-bot+netdevbpf
2026-03-24 23:00 ` Martin KaFai Lau [this message]
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