DPDK patches and discussions
 help / color / mirror / Atom feed
From: Jerin Jacob <jerin.jacob@caviumnetworks.com>
To: Takeshi Yoshimura <t.yoshimura8869@gmail.com>
Cc: dev@dpdk.org, stable@dpdk.org, olivier.matz@6wind.com,
	chaozhu@linux.vnet.ibm.com, konstantin.ananyev@intel.com
Subject: Re: [dpdk-dev] [PATCH] rte_ring: fix racy dequeue/enqueue in ppc64
Date: Tue, 17 Jul 2018 09:04:13 +0530
Message-ID: <20180717033410.GA3344@jerin> (raw)
In-Reply-To: <CALBPOTWYxCN3+DL_5Ozx3parn+hyo582BJgrjRzicPOEwR1CcA@mail.gmail.com>

-----Original Message-----
> Date: Tue, 17 Jul 2018 11:54:18 +0900
> From: Takeshi Yoshimura <t.yoshimura8869@gmail.com>
> To: Jerin Jacob <jerin.jacob@caviumnetworks.com>
> Cc: dev@dpdk.org, stable@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH] rte_ring: fix racy dequeue/enqueue in ppc64
> 

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
> >>

  reply	other threads:[~2018-07-17  3:34 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-07-12  2:44 Takeshi Yoshimura
2018-07-12 17:08 ` Jerin Jacob
2018-07-17  2:54   ` Takeshi Yoshimura
2018-07-17  3:34     ` Jerin Jacob [this message]
2021-03-24 21:45       ` [dpdk-dev] [dpdk-stable] " Thomas Monjalon
2021-03-28  1:00         ` Honnappa Nagarahalli
2021-06-16  7:14           ` [dpdk-dev] 回复: " Feifei Wang
2021-06-16 16:37             ` [dpdk-dev] " Honnappa Nagarahalli

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20180717033410.GA3344@jerin \
    --to=jerin.jacob@caviumnetworks.com \
    --cc=chaozhu@linux.vnet.ibm.com \
    --cc=dev@dpdk.org \
    --cc=konstantin.ananyev@intel.com \
    --cc=olivier.matz@6wind.com \
    --cc=stable@dpdk.org \
    --cc=t.yoshimura8869@gmail.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

DPDK patches and discussions

This inbox may be cloned and mirrored by anyone:

	git clone --mirror https://inbox.dpdk.org/dev/0 dev/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 dev dev/ https://inbox.dpdk.org/dev \
		dev@dpdk.org
	public-inbox-index dev

Example config snippet for mirrors.
Newsgroup available over NNTP:
	nntp://inbox.dpdk.org/inbox.dpdk.dev


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git