Re: [PATCH mm/bpf v2] mm: bpf: add find_vma_no_check() without lockdep_assert on mm->mmap_lock

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 8 Sep 2021 10:21:18 -0700 Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:

> > Again I am ignorant on the details so if you can clarify the following
> > it may help me and others to better understand the problem:
> > 
> > 1. Peter's patch appears to just take the same "fallback" path
> >    that would be taken if the trylock failed.
> >    Is this really a breakage or just loss of performance ?
> >    I would expect the latter, since it is called "fallback".
> 
> As Yonghong explained it's a user space breakage.
> User space tooling expects build_id to be available 99.999% of the time
> and that's what users observed in practice.
> They've built a bunch of tools on top of this feature.
> The data from these tools goes into various datacenter tables
> and humans analyze it later.
> So Peter's proposal is not acceptable. We don't want to get yelled at.
> 

I'm not understanding.  Peter said "this patch merely removes a
performance tweak" and you and Yonghong said "it breaks userspace". 
These assertions are contradictory!

Please describe the expected userspace-visible change from Peter's
patch in full detail?

And yes, it is far preferable that we resolve this by changing BPF to
be a better interface citizen, please.  Let's put those thinking caps on?