linux-kernel - Re: [RFC] Add BPF_SYNCHRONIZE bpf(2) command

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Message-ID: <20180716202301.GB160902@joelaf.mtv.corp.google.com>
Date:   Mon, 16 Jul 2018 13:23:01 -0700
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Daniel Colascione <dancol@...gle.com>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Lorenzo Colitti <lorenzo@...gle.com>,
        Chenbo Feng <fengc@...gle.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Alexei Starovoitov <ast@...com>,
        lkml <linux-kernel@...r.kernel.org>,
        Tim Murray <timmurray@...gle.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [RFC] Add BPF_SYNCHRONIZE bpf(2) command

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@...lfernandes.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