netdev - Re: [RFC PATCH net 1/2] [BUGFIX] bonding: use local function pointer of bond->recv_probe in bond_handle

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Date:	Tue, 11 Oct 2011 15:23:53 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	HAYASAKA Mitsuo <mitsuo.hayasaka.hu@...achi.com>
Cc:	Américo Wang <xiyou.wangcong@...il.com>,
	Jay Vosburgh <fubar@...ibm.com>,
	Andy Gospodarek <andy@...yhouse.net>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, yrl.pp-manager.tt@...achi.com
Subject: Re: [RFC PATCH net 1/2] [BUGFIX] bonding: use local function
 pointer of bond->recv_probe in bond_handle_frame

Le mardi 11 octobre 2011 à 22:02 +0900, HAYASAKA Mitsuo a écrit :
> Hi WANG Cong
> 
> Thank you for your comments.
> 
> (2011/10/07 22:24), Américo Wang wrote:
> > On Fri, Oct 7, 2011 at 8:49 PM, Mitsuo Hayasaka
> > <mitsuo.hayasaka.hu@...achi.com> wrote:
> >> The bond->recv_probe is called in bond_handle_frame() when
> >> a packet is received, but bond_close() sets it to NULL. So,
> >> a panic occurs when both functions work in parallel.
> >>
> >> Why this happen:
> >> After null pointer check of bond->recv_probe, an sk_buff is
> >> duplicated and bond->recv_probe is called in bond_handle_frame.
> >> So, a panic occurs when bond_close() is called between the
> >> check and call of bond->recv_probe.
> >>
> >> Patch:
> >> This patch uses a local function pointer of bond->recv_probe
> >> in bond_handle_frame(). So, it can avoid the null pointer
> >> dereference.
> >>
> > 
> > Hmm, I don't doubt it can fix the problem, I am wondering if
> > bond->recv_probe should be protected by bond->lock...
> 
> Indeed, in general any resources should be protected from the asynchronous
> workers.
> 
> At first, I thought it should be handled with lock protection, as well.
> However, I guess that using bond->lock on this kind of hot-path may
> introduces unnecessary overhead. In addition, this code works well
> without the strict lock protection. So, I think this change is the
> right way to fix it.

Maybe, but then ACCESS_ONCE() is needed to prevent compiler to
'optimize' the temporary variable.

Or use rcu_dereference() to make the whole thing really safe and self
documented.



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