From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Hunt, David" <david.hunt@intel.com>
Subject: Re: [PATCH] eal: fix bug in x86 cmpset
Date: Fri, 10 Feb 2017 10:39:18 +0000
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To: Thomas Monjalon <thomas.monjalon@6wind.com>,
 Nikhil Rao <nikhil.rao@intel.com>, bruce.richardson@intel.com,
 Konstantin Ananyev <konstantin.ananyev@intel.com>
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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.

Rgds,
Dave.