netdev - Re: netconsole refcount warning

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Date:   Wed, 12 Jul 2017 15:30:37 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Dave Jones <davej@...emonkey.org.uk>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: netconsole refcount warning

On Sun, Jul 9, 2017 at 4:57 PM, Dave Jones <davej@...emonkey.org.uk> wrote:
> The new refcount debugging code spews this twice during boot on my router..
>
>
> refcount_t: increment on 0; use-after-free.
> ------------[ cut here ]------------
> WARNING: CPU: 1 PID: 17 at lib/refcount.c:152 refcount_inc+0x2b/0x30
> CPU: 1 PID: 17 Comm: ksoftirqd/1 Not tainted 4.12.0-firewall+ #8
> task: ffff8801d4441ac0 task.stack: ffff8801d4450000
> RIP: 0010:refcount_inc+0x2b/0x30
> RSP: 0018:ffff8801d4456da8 EFLAGS: 00010046
> RAX: 000000000000002c RBX: ffff8801d4c3cf40 RCX: 0000000000000000
> RDX: 000000000000002c RSI: 0000000000000003 RDI: ffffed003a88adab
> RBP: ffff8801d4456da8 R08: 0000000000000003 R09: fffffbfff4afcb57
> R10: 0000000000000000 R11: fffffbfff4afcb58 R12: ffff8801d4c3c540
> R13: 0000000000000082 R14: ffff8801ce9c7ff8 R15: ffff8801ce9c8aa0
> FS:  0000000000000000(0000) GS:ffff8801d6a00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007fa2b803156e CR3: 00000001c405d000 CR4: 00000000000406e0
> Call Trace:
>  zap_completion_queue+0xad/0x1a0


Sigh... it is on purpose:

commit 8a455b087c9629b3ae3b521b4f1ed16672f978cc
Author: Jarek Poplawski <jarkao2@...il.com>
Date:   Thu Mar 20 16:07:27 2008 -0700

    netpoll: zap_completion_queue: adjust skb->users counter

    zap_completion_queue() retrieves skbs from completion_queue where they have
    zero skb->users counter.  Before dev_kfree_skb_any() it should be non-zero
    yet, so it's increased now.

    Reported-and-tested-by: Andrew Morton <akpm@...ux-foundation.org>
    Signed-off-by: Jarek Poplawski <jarkao2@...il.com>
    Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
    Signed-off-by: David S. Miller <davem@...emloft.net>

We need to review it now. :-/