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To: Thomas Monjalon <thomas.monjalon@6wind.com>
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Cc: Nikhil Rao <nikhil.rao@intel.com>, bruce.richardson@intel.com,
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From: "Hunt, David" <david.hunt@intel.com>
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Date: Fri, 10 Feb 2017 11:56:27 +0000
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Subject: Re: [dpdk-dev] [PATCH] eal: fix bug in x86 cmpset
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On 10/2/2017 10:53 AM, Thomas Monjalon 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.

Agreed. It's probably also worth noting that its only on the __PIC__ enabled
codepath so would have more of an affect on the distros.