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

[Thomas Monjalon <thomas.monjalon@6wind.com>]

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.