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From: Thomas Monjalon <thomas.monjalon@6wind.com>
To: Stephen Hemminger <stephen@networkplumber.org>, "Hunt,
David" <david.hunt@intel.com>, Nikhil Rao <nikhil.rao@intel.com>
Cc: bruce.richardson@intel.com,
Konstantin Ananyev <konstantin.ananyev@intel.com>, dev@dpdk.org
Date: Thu, 09 Mar 2017 16:39:55 +0100
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Subject: Re: [dpdk-dev] [PATCH] eal: fix bug in x86 cmpset
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2017-02-10 08:46, Stephen Hemminger:
> On Fri, 10 Feb 2017 11:53:06 +0100
> Thomas Monjalon <thomas.monjalon@6wind.com> wrote:
>
> > 2017-02-10 10:39, Hunt, David:
> > >
> > > On 9/2/2017 4:53 PM, Thomas Monjalon wrote:
> > > > 2016-11-06 22:09, Thomas Monjalon:
> > > >> 2016-09-29 18:34, Thomas Monjalon:
> > > >>> 2016-09-30 02:54, Nikhil Rao:
> > > >>>> The original code used movl instead of xchgl, this caused
> > > >>>> rte_atomic64_cmpset to use ebx as the lower dword of the source
> > > >>>> to cmpxchg8b instead of the lower dword of function argument "src".
> > > >>> Could you please start the explanation with a statement of
> > > >>> what is wrong from an user point of view?
> > > >>> It could help to understand how severe it is.
> > > >> Please, we need a clear explanation of the bug, and an acknowledgement.
> > > > Should we close this bug?
> > >
> > > I took a few minutes to look at this, and the issue can easily be
> > > reproduced with a small snippet of code.
> > > With the 'mov', the lower dword of the result is incorrect. This is
> > > resolved by using 'xchgl'.
> > >
> > > void main()
> > > {
> > > uint64_t a = 0xff000000ff;
> > >
> > > rte_atomic64_cmpset( &a, 0xff000000ff, 0xfa000000fa);
> > > printf("0x%lx\n", a);
> > > }
> > >
> > > When using 'mov', the result is 0xfa00000000
> > > When using 'xchgl', the result is 0xfa000000fa, as expected.
> >
> > This operation is used a lot in drivers for link status.
> >
> > I think we need to clearly explain what was the consequence of this bug.
>
>
> A bigger issue is why there are a huge number of copies of the same link code
> in drivers. Definitely should be common code. Also why is cmpset used here
> when a simple atomic_set would work as well for what was intended.
I'm surprised that there is no progress on this issue.