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

[Stephen Hemminger <stephen@networkplumber.org>]

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.