[dpdk-dev] [PATCH] doc: add multi-proc shared lib mempool note

Eads, Gage gage.eads at intel.com
Thu Jun 20 21:07:15 CEST 2019



> -----Original Message-----
> From: Andrew Rybchenko [mailto:arybchenko at solarflare.com]
> Sent: Thursday, June 20, 2019 1:01 PM
> To: Eads, Gage <gage.eads at intel.com>; dev at dpdk.org
> Cc: olivier.matz at 6wind.com; stable at dpdk.org
> Subject: Re: [PATCH] doc: add multi-proc shared lib mempool note
> 
> On 6/18/19 9:48 PM, Gage Eads wrote:
> > The mempool library assigns handler ops indexes based on the dynamic
> > load order of mempool handlers. Indexes are used so a mempool can be
> > used by multiple processes, but this only works if all processes agree
> > on the mapping from index to mempool handler.
> >
> > When using the '-d' argument, it's possible for different processes to
> > load mempool handlers in different orders, and thus have different
> > index->handler mappings. Using a mempool in multiple of such processes
> > index->will
> > result in undefined behavior.
> >
> > This commit adds a note to the mempool library programmer's guide
> > warning users against this.
> >
> > Fixes: 449c49b93a6b ("mempool: support handler operations")
> > Cc: stable at dpdk.org
> >
> > Signed-off-by: Gage Eads <gage.eads at intel.com>
> > ---
> >   doc/guides/prog_guide/mempool_lib.rst | 7 +++++++
> >   1 file changed, 7 insertions(+)
> >
> > diff --git a/doc/guides/prog_guide/mempool_lib.rst
> > b/doc/guides/prog_guide/mempool_lib.rst
> > index 52a569f57..4470f6b38 100644
> > --- a/doc/guides/prog_guide/mempool_lib.rst
> > +++ b/doc/guides/prog_guide/mempool_lib.rst
> > @@ -133,6 +133,13 @@ For applications that use ``rte_pktmbuf_create()``,
> there is a config setting
> >   (``RTE_MBUF_DEFAULT_MEMPOOL_OPS``) that allows the application to
> make use of
> >   an alternative mempool handler.
> >
> > +  .. note::
> > +
> > +    When running a DPDK application with shared libraries, mempool
> handler
> > +    shared objects specified with the '-d' EAL command-line parameter are
> > +    dynamically loaded. When running a multi-process application with
> shared
> > +    libraries, the -d arguments for mempool handlers *must be specified in
> the
> > +    same order for all processes* to ensure correct operation.
> >
> 
> One more empty line is required here, other than that:
> Acked-by: Andrew Rybchenko <arybchenko at solarflare.com>

Can do. Just for my understanding, why is the extra empty line required?

> 
> I think it is OK as a fix for stable branches.
> 
> In theory I think it is still technically possible to guarantee ops indices
> correctness using dedicated memzone (as Olivier suggested), but with sync
> up on
> rte_mempool_{get,set}_ops() (which should be called when EAL is
> initialized) and
> reordering. However, it requires API breakage to avoid returning of
> ops_index on register (since it may change in secondary process on resync
> when EAL is already initialized).



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