netdev - Re: [RFC] net: fix data race on sk

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Message-ID: <1442516594.7946.2.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Thu, 17 Sep 2015 12:03:14 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Dmitry Vyukov <dvyukov@...gle.com>
Cc:	davem@...emloft.net, edumazet@...gle.com,
	alexander.h.duyck@...hat.com, jiri@...nulli.us,
	hannes@...essinduktion.org, linus.luessing@...3.blue,
	willemb@...gle.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, andreyknvl@...gle.com,
	kcc@...gle.com, glider@...gle.com, paulmck@...ux.vnet.ibm.com,
	hboehm@...gle.com
Subject: Re: [RFC] net: fix data race on sk_buff after re-cloning

On Thu, 2015-09-17 at 20:44 +0200, Dmitry Vyukov wrote:
> KernelThreadSanitizer (KTSAN) reported the following race (on 4.2 rc2):
> 
> ThreadSanitizer: data-race in __copy_skb_header
...

> 	if (likely(atomic_read(&skb->users) == 1))
> 		smp_rmb();
> 
> The patch contains a proposed fix.
> If it looks good to you and the scenario looks sane,
> then I will update the description and resend it.
> ---
>  net/core/skbuff.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)

I have to double check this patch, but in the case it is needed,
it would be better to not use fancy new atomic_read_acquire(),
as backporting the fix up to 3.19 (where the bug was probably added)
will require extra hassle.

atomic_read_acquire() would be fine for cleanups and new code, in next
branch.

Thanks !


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