Re: [PATCH v9] eal: add seqlock

[Stephen Hemminger <stephen@networkplumber.org>]

On Wed, 1 Jun 2022 08:19:54 +0000
Mattias Rönnblom <mattias.ronnblom@ericsson.com> wrote:

> On 2022-06-01 00:45, Stephen Hemminger wrote:
> > On Mon, 23 May 2022 16:23:46 +0200
> > Mattias Rönnblom <mattias.ronnblom@ericsson.com> wrote:
> >   
> >> +/**
> >> + * The RTE seqcount type.
> >> + */
> >> +typedef struct {
> >> +	uint32_t sn; /**< A sequence number for the protected data. */
> >> +} rte_seqcount_t;  
> > 
> > Don't need structure for only one element.
> >   
> 
> The struct adds a degree of type safety, with no run-time cost.

Makes sense.

> 
> > typedef uint32_t rte_seqcount_t;
> > 
> > +	if (unlikely(begin_sn != end_sn))
> > +		return true;
> > +
> > +	return false;
> > 
> > Prefer to avoid conditional if possible (compiler will optimize it as):
> > 
> >          return begin_sn == end_sn;  
> 
> Is this a readability argument, or a performance one?
> 
> The compiler might use the unlikely hint to do something useful, like 
> avoiding a branch in the common case.

It is a matter of taste. I always prefer writing the smallest (within reason)
amount of code as possible. And my preference is to do things with declarative
and data statements rather than conditionals.