doc: add a note for multi-process in mempool guide

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 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@dpdk.org
Signed-off-by: Gage Eads <gage.eads@intel.com>
Acked-by: Andrew Rybchenko <arybchenko@solarflare.com>

diff --git a/doc/guides/prog_guide/mempool_lib.rst b/doc/guides/prog_guide/mempool_lib.rst
index 52a569f..3bb84b0 100644 (file)
--- a/doc/guides/prog_guide/mempool_lib.rst
+++ b/doc/guides/prog_guide/mempool_lib.rst
@@ -133,6 +133,14 @@ 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.
+
 
 Use Cases
 ---------