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Date:   Fri, 12 Oct 2018 19:31:40 -0700
From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
To:     Daniel Colascione <dancol@google.com>
Cc:     joelaf@google.com, linux-kernel@vger.kernel.org,
        timmurray@google.com, netdev@vger.kernel.org,
        Lorenzo Colitti <lorenzo@google.com>,
        Chenbo Feng <fengc@google.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Alexei Starovoitov <ast@fb.com>,
        Daniel Borkmann <daniel@iogearbox.net>
Subject: Re: [PATCH v4] Wait for running BPF programs when updating map-in-map
Message-ID: <20181013023138.m445q6itdmyxdtoc@ast-mbp.dhcp.thefacebook.com>
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 <20181012105427.243779-1-dancol@google.com>
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On Fri, Oct 12, 2018 at 03:54:27AM -0700, Daniel Colascione wrote:
> The map-in-map frequently serves as a mechanism for atomic
> snapshotting of state that a BPF program might record.  The current
> implementation is dangerous to use in this way, however, since
> userspace has no way of knowing when all programs that might have
> retrieved the "old" value of the map may have completed.
> 
> This change ensures that map update operations on map-in-map map types
> always wait for all references to the old map to drop before returning
> to userspace.
> 
> Signed-off-by: Daniel Colascione <dancol@google.com>
> ---
>  kernel/bpf/syscall.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 8339d81cba1d..d7c16ae1e85a 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -741,6 +741,18 @@ static int map_lookup_elem(union bpf_attr *attr)
>  	return err;
>  }
>  
> +static void maybe_wait_bpf_programs(struct bpf_map *map)
> +{
> +	/* Wait for any running BPF programs to complete so that
> +	 * userspace, when we return to it, knows that all programs
> +	 * that could be running use the new map value.
> +	 */
> +	if (map->map_type == BPF_MAP_TYPE_HASH_OF_MAPS ||
> +	    map->map_type == BPF_MAP_TYPE_ARRAY_OF_MAPS) {
> +		synchronize_rcu();
> +	}

extra {} were not necessary. I removed them while applying to bpf-next.
Please run checkpatch.pl next time.
Thanks