netdev - Re: [PATCH net] netlink: Don't reorder loads/stores before marking mmap netlink frame as available

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Message-ID: <CA+55aFyauxJFoK-ABmT+Zd3-oEV0d_FCXPLXrS_3yMG24Fe=8Q@mail.gmail.com>
Date:	Thu, 18 Dec 2014 11:13:08 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Thomas Graf <tgraf@...g.ch>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	David Miller <davem@...emloft.net>,
	Daniel Borkmann <dborkman@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	Patrick McHardy <kaber@...sh.net>,
	Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH net] netlink: Don't reorder loads/stores before marking
 mmap netlink frame as available

On Thu, Dec 18, 2014 at 2:30 AM, Thomas Graf <tgraf@...g.ch> wrote:
>
> diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
> index ef5f77b..2662821 100644
> --- a/net/netlink/af_netlink.c
> +++ b/net/netlink/af_netlink.c
> @@ -550,9 +550,9 @@ static enum nl_mmap_status netlink_get_status(const struct nl_mmap_hdr *hdr)
>  static void netlink_set_status(struct nl_mmap_hdr *hdr,
>                                enum nl_mmap_status status)
>  {
> +       smp_mb();
>         hdr->nm_status = status;
>         flush_dcache_page(pgvec_to_page(hdr));
> -       smp_wmb();
>  }

If this is performance-critical code (I have no idea), then a better
idea might be to use

        smp_store_release(&hdr->nm_status, status);

instead of using explicit memory barriers. That's going to  generally
be much cheaper than a memory barrier.

                      Linus
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