Date:   Mon, 9 Jul 2018 15:21:43 -0700
From:   Daniel Colascione <dancol@...gle.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Alexei Starovoitov <ast@...com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Tim Murray <timmurray@...gle.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        netdev <netdev@...r.kernel.org>, Chenbo Feng <fengc@...gle.com>
Subject: Re: [RFC] Add BPF_SYNCHRONIZE bpf(2) command

On Mon, Jul 9, 2018 at 3:10 PM, Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
> On Mon, Jul 09, 2018 at 02:36:37PM -0700, Daniel Colascione wrote:
>> On Mon, Jul 9, 2018 at 2:09 PM, Alexei Starovoitov
>> <alexei.starovoitov@...il.com> wrote:
>> > On Sun, Jul 08, 2018 at 04:54:38PM -0400, Mathieu Desnoyers wrote:
>> >> ----- On Jul 7, 2018, at 4:33 PM, Joel Fernandes joelaf@...gle.com wrote:
>> >>
>> >> > On Fri, Jul 06, 2018 at 07:54:28PM -0700, Alexei Starovoitov wrote:
>> >> >> On Fri, Jul 06, 2018 at 06:56:16PM -0700, Daniel Colascione wrote:
>> >> >> > BPF_SYNCHRONIZE waits for any BPF programs active at the time of
>> >> >> > BPF_SYNCHRONIZE to complete, allowing userspace to ensure atomicity of
>> >> >> > RCU data structure operations with respect to active programs. For
>> >> >> > example, userspace can update a map->map entry to point to a new map,
>> >> >> > use BPF_SYNCHRONIZE to wait for any BPF programs using the old map to
>> >> >> > complete, and then drain the old map without fear that BPF programs
>> >> >> > may still be updating it.
>> >> >> >
>> >> >> > Signed-off-by: Daniel Colascione <dancol@...gle.com>
>> >> >> > ---
>> >> >> >  include/uapi/linux/bpf.h |  1 +
>> >> >> >  kernel/bpf/syscall.c     | 14 ++++++++++++++
>> >> >> >  2 files changed, 15 insertions(+)
>> >> >> >
>> >> >> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
>> >> >> > index b7db3261c62d..4365c50e8055 100644
>> >> >> > --- a/include/uapi/linux/bpf.h
>> >> >> > +++ b/include/uapi/linux/bpf.h
>> >> >> > @@ -98,6 +98,7 @@ enum bpf_cmd {
>> >> >> >          BPF_BTF_LOAD,
>> >> >> >          BPF_BTF_GET_FD_BY_ID,
>> >> >> >          BPF_TASK_FD_QUERY,
>> >> >> > +        BPF_SYNCHRONIZE,
>> >> >> >  };
>> >> >> >
>> >> >> >  enum bpf_map_type {
>> >> >> > diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
>> >> >> > index d10ecd78105f..60ec7811846e 100644
>> >> >> > --- a/kernel/bpf/syscall.c
>> >> >> > +++ b/kernel/bpf/syscall.c
>> >> >> > @@ -2272,6 +2272,20 @@ SYSCALL_DEFINE3(bpf, int, cmd, union bpf_attr __user *,
>> >> >> > uattr, unsigned int, siz
>> >> >> >          if (sysctl_unprivileged_bpf_disabled && !capable(CAP_SYS_ADMIN))
>> >> >> >                  return -EPERM;
>> >> >> >
>> >> >> > +        if (cmd == BPF_SYNCHRONIZE) {
>> >> >> > +                if (uattr != NULL || size != 0)
>> >> >> > +                        return -EINVAL;
>> >> >> > +                err = security_bpf(cmd, NULL, 0);
>> >> >> > +                if (err < 0)
>> >> >> > +                        return err;
>> >> >> > +                /* BPF programs are run with preempt disabled, so
>> >> >> > +                 * synchronize_sched is sufficient even with
>> >> >> > +                 * RCU_PREEMPT.
>> >> >> > +                 */
>> >> >> > +                synchronize_sched();
>> >> >> > +                return 0;
>> >> >>
>> >> >> I don't think it's necessary. sys_membarrier() can do this already
>> >> >> and some folks use it exactly for this use case.
>> >> >
>> >> > Alexei, the use of sys_membarrier for this purpose seems kind of weird to me
>> >> > though. No where does the manpage say membarrier should be implemented this
>> >> > way so what happens if the implementation changes?
>> >> >
>> >> > Further, membarrier manpage says that a memory barrier should be matched with
>> >> > a matching barrier. In this use case there is no matching barrier, so it
>> >> > makes it weirder.
>> >> >
>> >> > Lastly, sys_membarrier seems will not work on nohz-full systems, so its a bit
>> >> > fragile to depend on it for this?
>> >> >
>> >> >        case MEMBARRIER_CMD_GLOBAL:
>> >> >                /* MEMBARRIER_CMD_GLOBAL is not compatible with nohz_full. */
>> >> >                if (tick_nohz_full_enabled())
>> >> >                        return -EINVAL;
>> >> >                if (num_online_cpus() > 1)
>> >> >                        synchronize_sched();
>> >> >                return 0;
>> >> >
>> >> >
>> >> > Adding Mathieu as well who I believe is author/maintainer of membarrier.
>> >>
>> >> See commit 907565337
>> >> "Fix: Disable sys_membarrier when nohz_full is enabled"
>> >>
>> >> "Userspace applications should be allowed to expect the membarrier system
>> >> call with MEMBARRIER_CMD_SHARED command to issue memory barriers on
>> >> nohz_full CPUs, but synchronize_sched() does not take those into
>> >> account."
>> >>
>> >> So AFAIU you'd want to re-use membarrier to issue synchronize_sched, and you
>> >> only care about kernel preempt off critical sections.
>> >>
>> >> Clearly bpf code does not run in user-space, so it would "work".
>> >>
>> >> But the guarantees provided by membarrier are not to synchronize against
>> >> preempt off per se. It's just that the current implementation happens to
>> >> do that. The point of membarrier is to turn user-space memory barriers
>> >> into compiler barriers.
>> >>
>> >> If what you need is to wait for a RCU grace period for whatever RCU flavor
>> >> ebpf is using, I would against using membarrier for this. I would rather
>> >> recommend adding a dedicated BPF_SYNCHRONIZE so you won't leak
>> >> implementation details to user-space, *and* you can eventually change you
>> >> RCU implementation for e.g. SRCU in the future if needed.
>> >
>> > The point about future changes to underlying bpf mechanisms is valid.
>> > There is work already on the way to reduce the scope of preempt_off+rcu_lock
>> > that currently lasts the whole prog. We will have new prog types that won't
>> > have such wrappers and will do rcu_lock/unlock and preempt on/off only
>> > when necessary.
>> > So something like BPF_SYNCHRONIZE will break soon, since the kernel cannot have
>> > guarantees on when programs finish. Calling this command BPF_SYNCHRONIZE_PROG
>> > also won't make sense for the same reason.
>> > What we can do it instead is to define synchronization barrier for
>> > programs accessing maps. May be call it something like:
>> > BPF_SYNC_MAP_ACCESS ?
>>
>> I'm not sure what you're proposing. In the case the commit message
>> describes, a user-space program that wants to "drain" a map needs to
>> be confident that the map won't change under it, even across multiple
>> bpf system calls on that map. One way of doing that is to ensure that
>> nothing that could possibly hold a reference to that map is still
>> running. Are you proposing some kind of refcount-draining approach?
>> Simple locking won't work, since BPF programs can't block, and I don't
>> see right now how a simple barrier would help.
>
> I'm proposing few changes for your patch:
> s/BPF_SYNCHRONIZE/BPF_SYNC_MAP_ACCESS/
> and s/synchronize_sched/synchronize_rcu/
> with detailed comment in uapi/bpf.h that has an example why folks
> would want to use this new cmd.

Thanks for clarifying.

> I think the bpf maps will be rcu protected for foreseeable future
> even when rcu_read_lock/unlock will be done by the programs instead of
> kernel wrappers.

Can we guarantee that we always obtain a map reference and dispose of
that reference inside the same critical section? If so, can BPF
programs then disable preemption for as long as they'd like?

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