netdev - Re: [PATCH bpf 0/4] bpf: remove __rcu annotations from bpf_prog

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Message-ID: <5ed25b81-fdd0-d707-f012-736fe6269a72@gmail.com>
Date:   Tue, 14 May 2019 19:44:30 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Stanislav Fomichev <sdf@...ichev.me>
Cc:     Stanislav Fomichev <sdf@...gle.com>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH bpf 0/4] bpf: remove __rcu annotations from bpf_prog_array

On 5/14/19 7:27 PM, Alexei Starovoitov wrote:

> what about activate_effective_progs() ?
> I wouldn't want to lose the annotation there.
> but then array_free will lose it?
> in some cases it's called without mutex in a destruction path.
> also how do you propose to solve different 'mtx' in
> lockdep_is_held(&mtx)); ?
> passing it through the call chain is imo not clean.
> 

Usage of RCU api in BPF is indeed a bit strange and lacks lockdep support.

Looking at bpf_prog_array_copy_core() for example, it looks like the __rcu
in the first argument is not needed, since the caller must have done the proper dereference already,
and the caller knows which mutex is protecting its rcu_dereference_protected() for the writer sides.

bpf_prog_array_copy_core() should manipulate standard pointers, with no __rcu stuff.

The analogy in net/ are probably the rtnl_dereference() users.