Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Mon, 16 Jul 2018 13:23:01 -0700
From:   Joel Fernandes <joel@joelfernandes.org>
To:     Daniel Colascione <dancol@google.com>
Cc:     Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        Lorenzo Colitti <lorenzo@google.com>,
        Chenbo Feng <fengc@google.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Joel Fernandes <joelaf@google.com>,
        Alexei Starovoitov <ast@fb.com>,
        lkml <linux-kernel@vger.kernel.org>,
        Tim Murray <timmurray@google.com>,
        Daniel Borkmann <daniel@iogearbox.net>,
        netdev <netdev@vger.kernel.org>
Subject: Re: [RFC] Add BPF_SYNCHRONIZE bpf(2) command
Message-ID: <20180716202301.GB160902@joelaf.mtv.corp.google.com>
References: <CAKOZuet+pjqM8zghmXYdA9f10bsYOzMuSQhSYXG5wbrNmFEqpQ@mail.gmail.com>
 <20180709221005.sintsjkle4xpkcyk@ast-mbp.dhcp.thefacebook.com>
 <CAKOZuetaKNaJVWcu10ZcsiXiK4X+-2z3eFaz1pF8DSxdQ_mEnQ@mail.gmail.com>
 <20180709223439.uc2a6hyic35inwye@ast-mbp.dhcp.thefacebook.com>
 <CAMOXUJ=QmkjTqbWqPRxb=R7f6EpsHBkDQ4MhrSD=VPiDTj7RiA@mail.gmail.com>
 <20180710235252.mioihpgtu4n3syaq@ast-mbp.dhcp.thefacebook.com>
 <CAKD1Yr07uxpd95oGAE7QxfrS_qWuiLcUmy7nsEsC0CzVhV2-rg@mail.gmail.com>
 <20180711034017.o2ehf27tv5hpl3td@ast-mbp.dhcp.thefacebook.com>
 <20180714181815.GA199777@joelaf.mtv.corp.google.com>
 <CAKOZuetwbE2HSa7vfVoVYwSqfz0ZcNWO93Y=pcJ-q4uEWHSuNg@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAKOZuetwbE2HSa7vfVoVYwSqfz0ZcNWO93Y=pcJ-q4uEWHSuNg@mail.gmail.com>
User-Agent: Mutt/1.9.2 (2017-12-15)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Mon, Jul 16, 2018 at 08:29:47AM -0700, Daniel Colascione wrote:
> On Sat, Jul 14, 2018 at 11:18 AM, Joel Fernandes <joel@joelfernandes.org> wrote:
> > On Tue, Jul 10, 2018 at 08:40:19PM -0700, Alexei Starovoitov wrote:
> > [..]
> >> > The kernel program might do:
> >> >
> >> > =====
> >> > const int current_map_key = 1;
> >> > void *current_map = bpf_map_lookup_elem(outer_map, &current_map_key);
> >> >
> >> > int stats_key = 42;
> >> > uint64_t *stats_value = bpf_map_lookup_elem(current_map, &stats_key);
> >> > __sync_fetch_and_add(&stats_value, 1);
> >> > =====
> >> >
> >> > If a userspace does:
> >> >
> >> > 1. Write new fd to outer_map[1].
> >> > 2. Call BPF_SYNC_MAP_ACCESS.
> >> > 3. Start deleting everything in the old map.
> >> >
> >> > How can we guarantee that the __sync_fetch_and_add will not add to the
> >> > old map?
> >>
> >> without any changes to the kernel sys_membarrier will work.
> >> And that's what folks use already.
> >> BPF_SYNC_MAP_ACCESS implemented via synchronize_rcu() will work
> >> as well whether in the current implementation where rcu_lock/unlock
> >> is done outside of the program and in the future when
> >> rcu_lock/unlock are called by the program itself.
> >
> > Cool Alexei and Lorenzo, sounds great to me. Daniel want to send a follow up
> > patch with BPF_SYNC_MAP_ACCESS changes then?
> 
> Will do. Mind if I just mine this thread for the doc comment?

Do you mean the changelog? Yes I believe you could use the discussion in this
thread for the rationale as Alexei described.

thanks,

- Joel