RE: [PATCH 4/4] mempool: use lcore API to check if lcore ID is valid

[Konstantin Ananyev <konstantin.ananyev@huawei.com>]

> 
> <snip>
> >
> >
> >
> > >
> > > Use lcore API to check if the lcore ID is valid. The runtime check
> > > does not add much value.
> >
> > From my perspective it adds a perfect value:
> > Only threads with valid lcore id have their own default mempool cache.
> The threads would call 'rte_lcore_id()' to return their lcore_id. This ensures the lcore_id is valid already.
> Why do we need to check it
> again in rte_mempool_default_cache? Why would a thread use an incorrect lcore_id?

rte_lcore_id() will just return you value of per-thread variable: RTE_PER_LCORE(_lcore_id).
Without any checking. For non-eal tthreads this value would be UINT32_MAX.
 
> >
> > > Hence use assert to validate
> > > the lcore ID.
> >
> > Wonder why?
> > What's wrong for the thread to try to get default mempool cache?
> What are the cases where a thread does not know that it is not an EAL thread and call rte_mempool_default_cache with a random
> lcore_id?
> Since, this API is called in the data plane, it makes sense to remove any additional validations.

Why is that?
I believe this API have to be generic and safe to call from any thread.
Let's look for example at:
*/
static __rte_always_inline void
rte_mempool_put_bulk(struct rte_mempool *mp, void * const *obj_table,
                     unsigned int n)
{
        struct rte_mempool_cache *cache;
        cache = rte_mempool_default_cache(mp, rte_lcore_id());
        rte_mempool_trace_put_bulk(mp, obj_table, n, cache);
        rte_mempool_generic_put(mp, obj_table, n, cache);
} 

Right now it is perfectly valid to invoke it from any thread.
With the change you propose invoking mempool_put() from non-EAL thread
will cause either a crash or silent memory corruption.
 
> > That would change existing behavior and in general seems wrong to me.
> Agree on the change in existing behavior. We can discuss this once we agree/disagree on the above.
> 
> > So I am strongly opposed.
> >
> > > Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
> > > Reviewed-by: Wathsala Vithanage <wathsala.vithanage@arm.com>
> > > Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
> > > ---
> > >  lib/mempool/rte_mempool.h | 5 ++---
> > >  1 file changed, 2 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/lib/mempool/rte_mempool.h b/lib/mempool/rte_mempool.h
> > > index 009bd10215..00c5aa961b 100644
> > > --- a/lib/mempool/rte_mempool.h
> > > +++ b/lib/mempool/rte_mempool.h
> > > @@ -1314,10 +1314,9 @@ rte_mempool_cache_free(struct
> > rte_mempool_cache
> > > *cache);  static __rte_always_inline struct rte_mempool_cache *
> > > rte_mempool_default_cache(struct rte_mempool *mp, unsigned lcore_id)
> > > {
> > > -	if (mp->cache_size == 0)
> > > -		return NULL;
> > > +	RTE_ASSERT(rte_lcore_id_is_valid(lcore_id));
> > >
> > > -	if (lcore_id >= RTE_MAX_LCORE)
> > > +	if (mp->cache_size == 0)
> > >  		return NULL;
> > >
> > >  	rte_mempool_trace_default_cache(mp, lcore_id,
> > > --
> > > 2.25.1
> > >