Subject: Re: [RFC] Add BPF_SYNCHRONIZE bpf(2) command

From: Mathieu Desnoyers <[email-masked]>
Date: Sun, 8 Jul 2018 16:54:38 -0400 (EDT)

----- On Jul 7, 2018, at 4:33 PM, Joel Fernandes [email-masked] 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 <[email-masked]>
>> > ---
>> >  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.

Thanks,

Mathieu

> thanks!
> - Joel

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com