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

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Date:   Tue, 14 May 2019 20:16:45 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Stanislav Fomichev <sdf@...ichev.me>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        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 Tue, May 14, 2019 at 07:56:36PM -0700, Stanislav Fomichev wrote:
> On 05/14, Eric Dumazet wrote:
> > 
> > 
> > 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?
> It would not have have it because the input is the result of
> bpf_prog_array_alloc() which returns kmalloc'd pointer (and
> is not bound to an rcu section).
> 
> > > in some cases it's called without mutex in a destruction path.
> Hm, can you point me to this place? I think I checked every path,
> maybe I missed something subtle. I'll double check.

I thought cgroup dying thingy is not doing it, but looks like it is.

> > > also how do you propose to solve different 'mtx' in
> > > lockdep_is_held(&mtx)); ?
> > > passing it through the call chain is imo not clean.
> Every caller would know which mutex protects it. As Eric said below,
> I'm adding a bunch of xxx_dereference macros that hardcode mutex, like
> the existing rtnl_dereference.

I have a hard time imagining how it will look without being a mess.
There are three mutexes to pass down instead of single rtnl_derefernce:
cgroup_mutex, ir_raw_handler_lock, bpf_event_mutex.

Anyway, let's see how the patches look and discuss further.