linux-kernel - Re: [PATCH net-next 05/12] ptr_ring: disallow lockless __ptr_ring

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Date:   Mon, 29 Jan 2018 06:41:18 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        John Fastabend <john.fastabend@...il.com>,
        David Miller <davem@...emloft.net>
Subject: Re: [PATCH net-next 05/12] ptr_ring: disallow lockless
 __ptr_ring_full

On Mon, Jan 29, 2018 at 11:36:09AM +0800, Jason Wang wrote:
> 
> 
> On 2018年01月26日 10:46, Michael S. Tsirkin wrote:
> > > On 2018年01月26日 07:36, Michael S. Tsirkin wrote:
> > > > Similar to bcecb4bbf88a ("net: ptr_ring: otherwise safe empty checks can
> > > > overrun array bounds") a lockless use of __ptr_ring_full might
> > > > cause an out of bounds access.
> > > > 
> > > > We can fix this, but it's easier to just disallow lockless
> > > > __ptr_ring_full for now.
> > > It looks to me that just fix this is better than disallow through doc (which
> > > is easily to be ignored ...).
> > > 
> > > Thanks
> > lockless is tricky, and I'd rather not sprinkle READ/WRITE_ONCE where
> > they aren't necessary.
> > 
> 
> The problem is then API looks a little bit strange. Lockless were only
> allowed to be done at __ptr_ring_empty() but not __ptr_ring_full().
> 
> Thanks

So __ptr_ring_empty doesn't really work lockless. It merely does not crash.
I don't believe we can do anything to remove the need to read the
docs unless people use the safe non __ variants.

-- 
MST