[dpdk-dev] [PATCH dpdk-dev v3] mempool: sort the rte_mempool_ops by name

[Andrew Rybchenko]

On 3/6/20 4:37 PM, Jerin Jacob wrote:
> On Fri, Mar 6, 2020 at 7:06 PM <xiangxia.m.yue at gmail.com> wrote:
>> From: Tonghao Zhang <xiangxia.m.yue at gmail.com>
>>
>> The order of mempool initiation affects mempool index in the
>> rte_mempool_ops_table. For example, when building APPs with:
>>
>> $ gcc -lrte_mempool_bucket -lrte_mempool_ring ...
>>
>> The "bucket" mempool will be registered firstly, and its index
>> in table is 0 while the index of "ring" mempool is 1. DPDK
>> uses the mk/rte.app.mk to build APPs, and others, for example,
>> Open vSwitch, use the libdpdk.a or libdpdk.so to build it.
>> The mempool lib linked in dpdk and Open vSwitch is different.
>>
>> The mempool can be used between primary and secondary process,
>> such as dpdk-pdump and pdump-pmd/Open vSwitch(pdump enabled).
>> There will be a crash because dpdk-pdump creates the "ring_mp_mc"
>> ring which index in table is 0, but the index of "bucket" ring
>> is 0 in Open vSwitch. If Open vSwitch use the index 0 to get
>> mempool ops and malloc memory from mempool. The crash will occur:
>>
>>      bucket_dequeue (access null and crash)
>>      rte_mempool_get_ops (should get "ring_mp_mc",
>>                           but get "bucket" mempool)
>>      rte_mempool_ops_dequeue_bulk
>>      ...
>>      rte_pktmbuf_alloc
>>      rte_pktmbuf_copy
>>      pdump_copy
>>      pdump_rx
>>      rte_eth_rx_burst
>>
>> To avoid the crash, there are some solution:
>> * constructor priority: Different mempool uses different
>>    priority in RTE_INIT, but it's not easy to maintain.
>>
>> * change mk/rte.app.mk: Change the order in mk/rte.app.mk to
>>    be same as libdpdk.a/libdpdk.so, but when adding a new mempool
>>    driver in future, we must make sure the order.
>>
>> * register mempool orderly: Sort the mempool when registering,
>>    so the lib linked will not affect the index in mempool table.
>>
>> Signed-off-by: Tonghao Zhang <xiangxia.m.yue at gmail.com>
>> Acked-by: Olivier Matz <olivier.matz at 6wind.com>
> Acked-by: Jerin Jacob <jerinj at marvell.com>

The patch is OK, but the fact that ops index changes during
mempool driver lifetime is frightening. In fact it breaks
rte_mempool_register_ops() return value semantics (read
as API break). The return value is not used in DPDK, but it
is a public function. If I'm not mistaken it should be taken
into account.

Also I remember patches which warn about above behaviour
in documentation. If behaviour changes, corresponding
documentation must be updated.