netdev - Re: [PATCH bpf-next] bpf: using rcu_read_lock for bpf_sk_storage

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Message-ID: <20200915083327.7e98cf2d@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Tue, 15 Sep 2020 08:33:27 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Yonghong Song <yhs@...com>
Cc:     <bpf@...r.kernel.org>, <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, <kernel-team@...com>,
        Martin KaFai Lau <kafai@...com>
Subject: Re: [PATCH bpf-next] bpf: using rcu_read_lock for
 bpf_sk_storage_map iterator

On Mon, 14 Sep 2020 11:46:30 -0700 Yonghong Song wrote:
> Currently, we use bucket_lock when traversing bpf_sk_storage_map
> elements. Since bpf_iter programs cannot use bpf_sk_storage_get()
> and bpf_sk_storage_delete() helpers which may also grab bucket lock,
> we do not have a deadlock issue which exists for hashmap when
> using bucket_lock ([1]).
> 
> If a bucket contains a lot of sockets, during bpf_iter traversing
> a bucket, concurrent bpf_sk_storage_{get,delete}() may experience
> some undesirable delays. Using rcu_read_lock() is a reasonable
> compromise here. Although it may lose some precision, e.g.,
> access stale sockets, but it will not hurt performance of other
> bpf programs.
> 
> [1] https://lore.kernel.org/bpf/20200902235341.2001534-1-yhs@fb.com
> 
> Cc: Martin KaFai Lau <kafai@...com>
> Signed-off-by: Yonghong Song <yhs@...com>

Sparse is not happy about it. Could you add some annotations, perhaps?

include/linux/rcupdate.h:686:9: warning: context imbalance in 'bpf_sk_storage_map_seq_find_next' - unexpected unlock
include/linux/rcupdate.h:686:9: warning: context imbalance in 'bpf_sk_storage_map_seq_stop' - unexpected unlock