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From:   Daniel Colascione <dancol@google.com>
Date:   Mon, 16 Jul 2018 08:29:47 -0700
Message-ID: <CAKOZuetwbE2HSa7vfVoVYwSqfz0ZcNWO93Y=pcJ-q4uEWHSuNg@mail.gmail.com>
Subject: Re: [RFC] Add BPF_SYNCHRONIZE bpf(2) command
To:     Joel Fernandes <joel@joelfernandes.org>
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>
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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?