netdev - Re: [PATCH] ptr_ring: add barriers

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Message-Id: <20171208.130848.1881550210595407116.davem@davemloft.net>
Date:   Fri, 08 Dec 2017 13:08:48 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     mst@...hat.com
Cc:     linux-kernel@...r.kernel.org, george.cherian@...ium.com,
        jasowang@...hat.com, edumazet@...gle.com, netdev@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH] ptr_ring: add barriers

From: "Michael S. Tsirkin" <mst@...hat.com>
Date: Tue, 5 Dec 2017 21:29:37 +0200

> Users of ptr_ring expect that it's safe to give the
> data structure a pointer and have it be available
> to consumers, but that actually requires an smb_wmb
> or a stronger barrier.
> 
> In absence of such barriers and on architectures that reorder writes,
> consumer might read an un=initialized value from an skb pointer stored
> in the skb array.  This was observed causing crashes.
> 
> To fix, add memory barriers.  The barrier we use is a wmb, the
> assumption being that producers do not need to read the value so we do
> not need to order these reads.
> 
> Reported-by: George Cherian <george.cherian@...ium.com>
> Suggested-by: Jason Wang <jasowang@...hat.com>
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> ---
> 
> George, could you pls report whether this patch fixes
> the issue for you?
> 
> This seems to be needed in stable as well.

I really need some testing feedback for this before I apply it
and queue it up for -stable.

George?