Re: [PATCH bpf 0/4] bpf: remove __rcu annotations from bpf_prog_array

[Eric Dumazet]

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.