Re: [PATCH v6 1/9] eal: generic 64 bit counter

[Stephen Hemminger <stephen@networkplumber.org>]

On Fri, 17 May 2024 08:44:42 +0200
Morten Brørup <mb@smartsharesystems.com> wrote:

> > From: Honnappa Nagarahalli [mailto:Honnappa.Nagarahalli@arm.com]
> > Sent: Friday, 17 May 2024 06.27
> > 
> > + Bruce for feedback on x86 architecture
> >   
> > > On May 16, 2024, at 10:30 PM, Stephen Hemminger <stephen@networkplumber.org>  
> > wrote:  
> > >
> > > On Fri, 17 May 2024 02:45:12 +0000
> > > Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com> wrote:
> > >  
> > >>> + * A counter is 64 bit value that is safe from split read/write
> > >>> + * on 32 bit platforms. It assumes that only one cpu at a time  
> > >> If we are defining the counter in this manner, then implementation cannot  
> > be generic. I think architectures will have constraints if they have to ensure
> > the 64b variables are not split.  
> > >>
> > >> I think we at least need the counter to be aligned on 8B boundary to have  
> > generic code.  
> > >
> > > The C standard has always guaranteed that read and write to unsigned log  
> > will not be split.
> > As I understand, this is true only if the variable is an atomic variable. If
> > not, there is no difference between atomic variables and non-atomic variables.
> >   
> > > Therefore if arch is 64 bit native there is no need for atomics  
> > At least on ARM architecture, if the variable is not aligned on 8B boundary,
> > the load or store are not atomic. I am sure it is the same on other
> > architectures.  

After reading this: Who's afraid of a big bad optimizing compiler?
 https://lwn.net/Articles/793253/

Looks like you are right, and atomic or read/write once is required.
Perhaps introducing rte_read_once and rte_write_once is good idea?
Several drivers are implementing it already.