DPDK seqlock

[Stephen Hemminger]

On Thu, 24 Mar 2022 04:52:07 +0000
Honnappa Nagarahalli <Honnappa.Nagarahalli at arm.com> wrote:

> <snip>
> 
> > 
> > Hi Mattias,
> >   
> > >
> > > Would it make sense to have a seqlock implementation in DPDK?  
> I do not have any issues with adding the seqlock to DPDK.
> 
> However, I am interested in understanding the use case. As I understand, seqlock is a type of reader-writer lock. This means that it is possible that readers (data plane) may be blocked till the writer completes the updates. Does not this mean, data plane might drop packets while the writer is updating entries?
> 
> > >
> > > I think so, since it's a very useful synchronization primitive in data
> > > plane applications.
> > >  
> > 
> > Agree, it might be useful.
> > As I remember rte_hash '_lf' functions do use something similar to seqlock, but
> > in hand-made manner.
> > Probably some other entities within DPDK itself or related projects will benefit
> > from it too...
> > 
> > Konstantin

As inventor of seqlock, it is really just a kind of reader/writer spinlock
where spinning trys to do useful work.  It useful for cases where the data
being accessed is too large for __atomic primitives.