[dpdk-dev] 回复: [dpdk-stable] [PATCH] rte_ring: fix racy dequeue/enqueue in ppc64

[Feifei Wang <Feifei.Wang2@arm.com>]

Hi, everyone

This patch can be closed with the following reasons.

> -----邮件原件-----
> 发件人: dev <dev-bounces@dpdk.org> 代表 Honnappa Nagarahalli
> 发送时间: 2021年3月28日 9:00
> 收件人: thomas@monjalon.net; Takeshi Yoshimura
> <t.yoshimura8869@gmail.com>
> 抄送: stable@dpdk.org; dev@dpdk.org; olivier.matz@6wind.com;
> chaozhu@linux.vnet.ibm.com; konstantin.ananyev@intel.com; Jerin Jacob
> <jerin.jacob@caviumnetworks.com>; nd <nd@arm.com>; nd <nd@arm.com>
> 主题: Re: [dpdk-dev] [dpdk-stable] [PATCH] rte_ring: fix racy
> dequeue/enqueue in ppc64
> 
> <snip>
> 
> > Subject: Re: [dpdk-stable] [dpdk-dev] [PATCH] rte_ring: fix racy
> > dequeue/enqueue in ppc64
> >
> > No reply after more than 2 years.
> > Unfortunately it is probably outdated now.
> > Classified as "Changes Requested".
> Looking at the code, I think this patch in fact fixes a bug. Appreciate rebasing
> this patch.
> 
> The problem is already fixed in '__rte_ring_move_cons_head' but needs to
> be fixed in '__rte_ring_move_prod_head'.
> This problem is fixed for C11 version due to acquire load of cons.tail and
> prod.tail.

First, for consumer in dequeue:
the reason for that adding a rmb in move_cons_head of “generic” is based on this patch:
http://patches.dpdk.org/project/dpdk/patch/1552409933-45684-2-git-send-email-gavin.hu@arm.com/

Slot                        Consumer                                                           Producer
1                     dequeue elements
2                                                                                                  update prod_tail
3                   load new prod_tail
4                   check room is enough(n < entries)

Dequeue elements maybe before load updated prod_tail, so consumer can load incorrect elements value.
For dequeue multiple consumers case, ‘rte_atomic32_cmpset’ with acquire and release order can prevent
dequeue before load prod_tail, no extra rmb is needed.

Second, for single producer in enqueue:

Slot                        Producer                                                             Consumer
1                     enqueue elements(not commited)
2                                                                                                  update consumer_tail
3                   load new consumer_tail
4                   check room is enough(n < entries)
5                   enqueued elements is committed

Though enqueue elements maybe reorder before load consumer_tail, these elements will not be committed until
‘check’ has finished. So from load to write control dependency is reliable and rmb is not needed here.
[1] https://www.cl.cam.ac.uk/~pes20/ppc-supplemental/test7.pdf (page:15)

As a result, it is unnecessary to add a rmb for enqueue single producer due to control dependency. And this patch can
be closed.

Best Regards
Feifei
> 
> >
> >
> > 17/07/2018 05:34, Jerin Jacob:
> > > From: Takeshi Yoshimura <t.yoshimura8869@gmail.com>
> > >
> > > Cc: olivier.matz@6wind.com
> > > Cc: chaozhu@linux.vnet.ibm.com
> > > Cc: konstantin.ananyev@intel.com
> > >
> > > >
> > > > > Adding rte_smp_rmb() cause performance regression on non x86
> > platforms.
> > > > > Having said that, load-load barrier can be expressed very  well
> > > > > with C11 memory model. I guess ppc64 supports C11 memory model.
> > > > > If so, Could you try CONFIG_RTE_RING_USE_C11_MEM_MODEL=y
> for
> > > > > ppc64 and check original issue?
> > > >
> > > > Yes, the performance regression happens on non-x86 with single
> > > > producer/consumer.
> > > > The average latency of an enqueue was increased from 21 nsec to 24
> > > > nsec in my simple experiment. But, I think it is worth it.
> > >
> > > That varies to machine to machine. What is the burst size etc.
> > >
> > > >
> > > >
> > > > I also tested C11 rte_ring, however, it caused the same race
> > > > condition in
> > ppc64.
> > > > I tried to fix the C11 problem as well, but I also found the C11
> > > > rte_ring had other potential incorrect choices of memory orders,
> > > > which caused another race condition in ppc64.
> > >
> > > Does it happens on all ppc64 machines? Or on a specific machine?
> > > Is following tests are passing on your system without the patch?
> > > test/test/test_ring_perf.c
> > > test/test/test_ring.c
> > >
> > > >
> > > > For example,
> > > > __ATOMIC_ACQUIRE is passed to __atomic_compare_exchange_n(),
> but I
> > > > am not sure why the load-acquire is used for the compare exchange.
> > >
> > > It correct as per C11 acquire and release semantics.
> > >
> > > > Also in update_tail, the pause can be called before the data copy
> > > > because of ht->tail load without atomic_load_n.
> > > >
> > > > The memory order is simply difficult, so it might take a bit
> > > > longer time to check if the code is correct. I think I can fix the
> > > > C11 rte_ring as another patch.
> > > >
> > > > >>
> > > > >> SPDK blobfs encountered a crash around rte_ring dequeues in ppc64.
> > > > >> It uses a single consumer and multiple producers for a rte_ring.
> > > > >> The problem was a load-load reorder in
> rte_ring_sc_dequeue_bulk().
> > > > >
> > > > > Adding rte_smp_rmb() cause performance regression on non x86
> > platforms.
> > > > > Having said that, load-load barrier can be expressed very  well
> > > > > with C11 memory model. I guess ppc64 supports C11 memory model.
> > > > > If so, Could you try CONFIG_RTE_RING_USE_C11_MEM_MODEL=y
> for
> > > > > ppc64 and check original issue?
> > > > >
> > > > >>
> > > > >> The reordered loads happened on r->prod.tail in
> > >
> > > There is rte_smp_rmb() just before reading r->prod.tail in
> > >         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> > > _rte_ring_move_cons_head(). Would that not suffice the requirement?
> > >
> > > Can you check adding compiler barrier and see is compiler is
> > > reordering the stuff?
> > >
> > > DPDK's ring implementation is based freebsd's ring implementation, I
> > > don't see need for such barrier
> > >
> > > https://github.com/freebsd/freebsd/blob/master/sys/sys/buf_ring.h
> > >
> > > If it is something specific to ppc64 or a specific ppc64 machine, we
> > > could add a compile option as it is arch specific to avoid
> > > performance impact on other architectures.
> > >
> > > > >> __rte_ring_move_cons_head() (rte_ring_generic.h) and ring[idx]
> > > > >> in
> > > > >> DEQUEUE_PTRS() (rte_ring.h). They have a load-load control
> > > > >> dependency, but the code does not satisfy it. Note that they
> > > > >> are not reordered if __rte_ring_move_cons_head() with is_sc !=
> > > > >> 1 because cmpset invokes a read barrier.
> > > > >>
> > > > >> The paired stores on these loads are in ENQUEUE_PTRS() and
> > > > >> update_tail(). Simplified code around the reorder is the following.
> > > > >>
> > > > >> Consumer             Producer
> > > > >> load idx[ring]
> > > > >>                      store idx[ring]
> > > > >>                      store r->prod.tail load r->prod.tail
> > > > >>
> > > > >> In this case, the consumer loads old idx[ring] and confirms the
> > > > >> load is valid with the new r->prod.tail.
> > > > >>
> > > > >> I added a read barrier in the case where __IS_SC is passed to
> > > > >> __rte_ring_move_cons_head(). I also fixed
> > > > >> __rte_ring_move_prod_head() to avoid similar problems with a
> > > > >> single
> > producer.
> > > > >>
> > > > >> Cc: stable@dpdk.org
> > > > >>
> > > > >> Signed-off-by: Takeshi Yoshimura <tyos@jp.ibm.com>
> > > > >> ---
> > > > >>  lib/librte_ring/rte_ring_generic.h | 10 ++++++----
> > > > >>  1 file changed, 6 insertions(+), 4 deletions(-)
> > > > >>
> > > > >> diff --git a/lib/librte_ring/rte_ring_generic.h
> > > > >> b/lib/librte_ring/rte_ring_generic.h
> > > > >> index ea7dbe5b9..477326180 100644
> > > > >> --- a/lib/librte_ring/rte_ring_generic.h
> > > > >> +++ b/lib/librte_ring/rte_ring_generic.h
> > > > >> @@ -90,9 +90,10 @@ __rte_ring_move_prod_head(struct rte_ring
> > > > >> *r,
> > unsigned int is_sp,
> > > > >>                         return 0;
> > > > >>
> > > > >>                 *new_head = *old_head + n;
> > > > >> -               if (is_sp)
> > > > >> +               if (is_sp) {
> > > > >> +                       rte_smp_rmb();
> > > > >>                         r->prod.head = *new_head, success = 1;
> > > > >> -               else
> > > > >> +               } else
> > > > >>                         success = rte_atomic32_cmpset(&r->prod.head,
> > > > >>                                         *old_head, *new_head);
> > > > >>         } while (unlikely(success == 0)); @@ -158,9 +159,10 @@
> > > > >> __rte_ring_move_cons_head(struct rte_ring *r, unsigned int is_sc,
> > > > >>                         return 0;
> > > > >>
> > > > >>                 *new_head = *old_head + n;
> > > > >> -               if (is_sc)
> > > > >> +               if (is_sc) {
> > > > >> +                       rte_smp_rmb();
> > > > >>                         r->cons.head = *new_head, success = 1;
> > > > >> -               else
> > > > >> +               } else
> > > > >>                         success = rte_atomic32_cmpset(&r->cons.head,
> *old_head,
> > > > >>                                         *new_head);
> > > > >>         } while (unlikely(success == 0));
> > > > >> --
> > > > >> 2.17.1
> >
> >