Re: [PATCH v2] Add BPF_SYNCHRONIZE_MAPS bpf(2) command

[Joel Fernandes]

On Sun, Jul 29, 2018 at 06:51:18PM +0300, Alexei Starovoitov wrote:
> On Thu, Jul 26, 2018 at 7:51 PM, Daniel Colascione <dancol@xxxxxxxxxx> wrote:
> > BPF_SYNCHRONIZE_MAPS waits for the release of any references to a BPF
> > map made by a BPF program that is running at the time the
> > BPF_SYNCHRONIZE_MAPS command is issued. The purpose of this command is
> > to provide a means for userspace to replace a BPF map with another,
> > newer version, then ensure that no component is still using the "old"
> > map before manipulating the "old" map in some way.
> >
> > Signed-off-by: Daniel Colascione <dancol@xxxxxxxxxx>
> > ---
> >  include/uapi/linux/bpf.h |  9 +++++++++
> >  kernel/bpf/syscall.c     | 13 +++++++++++++
> >  2 files changed, 22 insertions(+)
> >
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index b7db3261c62d..5b27e9117d3e 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -75,6 +75,14 @@ struct bpf_lpm_trie_key {
> >         __u8    data[0];        /* Arbitrary size */
> >  };
> >
> > +/* BPF_SYNCHRONIZE_MAPS waits for the release of any references to a
> > + * BPF map made by a BPF program that is running at the time the
> > + * BPF_SYNCHRONIZE_MAPS command is issued. The purpose of this command
> 
> that doesn't sound right to me.
> such command won't wait for the release of the references.
> in case of map-in-map the program does not hold
> the references to inner map (only to outer map).

I didn't follow this completely.

The userspace program is using the inner map per your description of the
algorithm for using map-in-map to solve the race conditions that this patch
is trying to address:

If you don't mind, I copy-pasted it below from your netdev post:

if you use map-in-map you don't need extra boolean map.
0. bpf prog can do
   inner_map = lookup(map_in_map, key=0);
   lookup(inner_map, your_real_key);
1. user space writes into map_in_map[0] <- FD of new map
2. some cpus are using old inner map and some a new
3. user space does sys_membarrier(CMD_GLOBAL) which will do synchronize_sched()
   which in CONFIG_PREEMPT_NONE=y servers is the same as synchronize_rcu()
   which will guarantee that progs finished.
4. scan old inner map

In step 2, as you mentioned there are CPUs using different inner maps. So
could you clarify how the synchronize_rcu mechanism will even work if you're
now saying "program does not hold references to the inner maps"? 

Thanks!

- Joel