[dpdk-dev] [PATCH] eal: fix bug in x86 cmpset

Hunt, David david.hunt at intel.com
Fri Feb 10 11:39:18 CET 2017


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.



More information about the dev mailing list