RE: [PATCH v1 3/4] ring: fix potential sync issue between head and tail values

Konstantin Ananyev
2025-06-02T11:11:37Z

Hi Wathsala,

> >
> > Thanks for the clarification, I now see your concern.
> >
> > So, in summary what you are saying is that tail update from
> > Thread #1 that happened at T0 is not observed by the thread #2
> > at T2 when it computed new_head and entries calculation.

Yes

> > That cannot happen in Arm v8/v9 because tail update is a
> > store-release and a load-acquire that program order follows it
> > can only be issued after all the cores have observed the
> > store-release (there is a synchronization with relationship to
> > store-release and load-acquire pairs).
> >
> > In the example you have provided Thread #1's
> > store(&prod.tail, 2, release) is observed by all the cores in the
> > system by the time same thread performs load(&prod.tail, acquire)
> > at T2. As per Arm v8/v9 memory model Thread #2 should observe
> > prod.tail=2 (not prod.tail=1).
> >
> > Arm Architecture Reference Manual section B2.10.11 states…
> >
> > "Where a Load-Acquire appears in program order after a Store-Release,
> > the memory access generated by the Store-Release instruction is
> > observed by each PE to the extent that PE is required to observe the
> > access coherently, before the memory access generated by the
> > Load-Acquire instruction is observed by that PE, to the extent that the
> > PE is required to observe the access coherently."

Interesting, thanks for pointing to that.
Indeed, according to the docs, acquire/release operations use a sequentially consistent model.
But we not always getting pure LDAR instruction here, it depends on compiler march/mcpu flags.
For some HW arch gcc (and clang) generate LDAR instruction, for others - LDPAPR.
And according to the doc:
https://developer.arm.com/documentation/102336/0100/Load-Acquire-and-Store-Release-instructions
for Load-AcquirePC the requirement that Load-Acquires are observed in order with preceding Store-Releases is dropped.
To be more specific here, on my box:
1) meson with -Dmachine=default (gcc options: "-march=armv8-a+crc") generates code with LDAR instruction
https://godbolt.org/z/5cEscdxrb
2) meson with -Dmachine=native (gcc options: " -mcpu=neoverse-n1") generates code with LDAPR instruction
https://godbolt.org/z/8KjcvYxch

soring_stress_test for 1) (LDAR) passes
soring_stress_test for 2) (LDAPR) fails with symptoms I described earlier

>
> In soring is this the pair that update the tail and move head?
>
> __rte_soring_update_tail:
> __rte_ring_update_tail(&rht->ht, head, next, st, enq);

That function is used to update prod.tail and cons.tail

> __rte_soring_move_cons_head:
> __rte_ring_headtail_move_head(&r->cons.ht, &r->stage[stage].ht, 0, ...);
>

__rte_ring_headtail_move_head is invoked to update prod.head (by __rte_soring_move_prod_head)
and cons.head (by __rte_soring_move_cons_head).

> If so, &rht->ht and &r->stage[stage].ht are the same address?

No.
In conventional ring we have just two stages: 'prod' and 'cons'.
So moving prod.head depends on cons.tail and visa-versa: moving cons.head depends on prod.tail.
With soring we have multiple (N) extra stages in between.
So the dependency chain is a bit longer:
prod.head depends on cons.tail
stage[0].head depends on prod.tail,
stage[1].head depends on stage[0].tail
...
stage{N-1].head depends on stage[N-2].tail
cons.head depends on stage[N-1].tail

> If they are
> not, then you will run into the issue you have seen (a.k.a "Other-multi-copy
> atomic" which is legit in Arm v8 and above).

Can you probably elaborate a bit more for me here?
Thanks
Konstantin

> Thanks.
>
> --wathsala
>
>
> > > -----Original Message-----
> > > From: Konstantin Ananyev <konstantin.ananyev@huawei.com>
> > > Sent: Monday, May 26, 2025 6:54 AM
> > > To: Wathsala Wathawana Vithanage <wathsala.vithanage@arm.com>;
> > > dev@dpdk.org
> > > Cc: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>;
> > > jerinj@marvell.com; hemant.agrawal@nxp.com; drc@linux.ibm.com; nd
> > > <nd@arm.com>
> > > Subject: RE: [PATCH v1 3/4] ring: fix potential sync issue between head and
> > tail
> > > values
> > >
> > > Hi Wathsala,
> > >
> > > >
> > > > Hi Konstanin,
> > > >
> > > > In rte_ring the store-release on tail update guarantees that CAS
> > > > won't get reordered with the store-released of the tail update.
> > > >
> > > > So, the sequence of events would look like this (combined view
> > > > of head and tail update)
> > > >
> > > > Releaxed-load(new_head, N) ----------------> (A)
> > > > Relaxed-CAS(d->head, new_head, old_head) ----------------> (B)
> > > > Store-release-store(d->tail, new_head) ----------------> (C)
> > > >
> > > > If we look at address dependencies, then...
> > > >
> > > > (B) depends on (A) due to new_head address dependency.
> > > > (C) depends on (A) due to new_head address dependency.
> > > >
> > > > So, dependency graph looks like this
> > > > (A)
> > > > / \
> > > > v v
> > > > (B) (C)
> > > >
> > > > There is no implicit dependence between (B) and (C), I think
> > > > this is the issue you are brining up.
> > > > Even though there is no dependence between the two,
> > > > the store-release of (C) ensures that (B) won't drop below it.
> > > > Therefore, the above graph can be turned into an ordered
> > > > sequence as shown below..
> > > >
> > > > (A) -> (B) -> (C)
> > >
> > > I do agree that with current implementation of
> > > __rte_ring_headtail_move_head()
> > > in lib/ring/rte_ring_c11_pvt.h the order of these 3 operations (A->B->C)
> > > should be sequential.
> > > The problem I am talking about is a different one:
> > > thread can see 'latest' 'cons.head' value, with 'previous' value for 'prod.tail' or
> > > visa-versa.
> > > In other words: 'cons.head' value depends on 'prod.tail', so before making
> > > latest 'cons.head'
> > > value visible to other threads, we need to ensure that latest 'prod.tail' is also
> > > visible.
> > > Let me try to explain it on the example:
> > >
> > > Suppose at some moment we have:
> > > prod={.head=2,.tail=1};
> > > cons={.head=0,.tail=0};
> > > I.e. thead #1 is in process to enqueue one more element into the ring.
> > >
> > > Thread #1 Thread #2
> > > T0:
> > > store(&prod.tail, 2, release);
> > > /*AFAIU: this is somewhat equivalent to: wmb(); prod.tail=2;
> > > * I.E. - it guarantees that all stores initiated before that operation will be
> > > visible
> > > * by other threads at the same moment or before new value of prod.tail wlll
> > > become
> > > * visible, but it doesn't guarantee that new prod.tail value will be visible to
> > > other
> > > * threads immediately.
> > > */
> > > ...
> > > move_cons_head(...,n=2)
> > move_cons_head(...,n=1)
> > > ... ...
> > > T1:
> > > *old_head = load(&cons.head, relaxed);
> > > fence(acquire);
> > > /*old_head == 0, no surprises */
> > > stail = load(&prod.tail, acquire);
> > > /*stail == 2, no surprises */
> > > *entries = (capacity + stail - *old_head);
> > > *new_head = *old_head + n;
> > > /* *entries == (2 - 0) == 2; *new_head==2; all good */
> > > ...
> > > T2:
> > > *old_head =
> > > load(&cons.head, relaxed);
> > > fence(acquire);
> > > /*old_head == 0, no
> > surprises
> > > */
> > > stail = load(&prod.tail,
> > > acquire);
> > > /* !!!!! stail == 1 !!!!! for Thread 2
> > > * Even though we do use acquire here - there was no *release* after
> > > store(&prod.tail).
> > > * So, still no guarantee that Thread 2 will see latest prod.tail value.
> > > */
> > > *entries = (capacity + stail -
> > > *old_head);
> > > /* *entries == (1 - 0) == 1,
> > still
> > > ok */
> > > *new_head = *old_head + n;
> > > /* *new_head == 1 */
> > > T3:
> > > success = CAS(&cons.head,
> > > old_head /*==0/, *new_head /*==2*/,
> > > relaxed, relaxed);
> > > /*success==1, cons.head==2*/
> > > ...
> > > T4:
> > > success = CAS(&cons.head,
> > > old_head /*==0/, *new_head
> > > /*==1*/,
> > > relaxed, relaxed);
> > > /*success==0, *old_head==2*/
> > > /* CAS() failed and provided Thread 2 with latest valued for cons.head(==2)
> > > * Thread 2 repeats attempt, starts second iteration
> > > */
> > > fence(acquire);
> > > stail = load(&prod.tail, acquire);
> > > /* !!!!! stail == 1 !!!!! for Thread 2
> > > * Still no *release* had happened after store(&prod.tail) at T0.
> > > * So, still no guarantee that Thread 2 will see latest prod.tail value.
> > > */
> > > *entries = (capacity + stail -
> > > *old_head);
> > > *new_head = *old_head + n;
> > >
> > > /* !!!!! *entries == (1 - 2) == -1(UINT32_MAX); *new_head==(2+1)==3; !!!!!
> > > * we are about to corrupt our ring !!!!!
> > > */
> > >
> > > >
> > > > I haven't looked at the so-ring yet. Could it be possible that the
> > > > issue is due to something else introduced in that code?
> > >
> > > Well, as I said, so far I wasn't able to re-produce this problem with
> > > conventional
> > > ring (ring_stress_autotest), only soring_stress_autotest is failing and
> > > for now - only on one specific ARM platform.
> > > Regarding soring specific fix (without touching common code) -
> > > sure it is also possible, pls see patch #2.
> > > There I just added 'fence(release);' straight after 'store(&tail);'
> > > That's seems enough to fix that problem within the soring only.
> > > Though, from my understanding rte_ring might also be affected,
> > > that's why I went ahead and prepared that patch.
> > > If you feel, that I a missing something here - pls shout.
> > > Konstantin
> > >
> > >
> > > > Thanks,
> > > >
> > > > --wathsala
> > > >
> > > > > This patch aims several purposes:
> > > > > - provide an alternative (and I think a better) way to fix the
> > > > > issue discussed in previous patch:
> > > > > "ring/soring: fix synchronization issue between head and tail values"
> > > > > - make sure that such problem wouldn’t happen within other usages of
> > > > > __rte_ring_headtail_move_head() – both current rte_ring
> > > > > implementation and possible future use-cases.
> > > > > - step towards unification of move_head() implementations and
> > > > > removing rte_ring_generic_pvt.h
> > > > > It uses Acquire-Release memory ordering for CAS operation in
> > > move_head().
> > > > > That guarantees that corresponding ‘tail’ updates will be visible before
> > > current
> > > > > ‘head’ is updated.
> > > > > As I said before: I think that in theory the problem described in previous
> > > patch
> > > > > might happen with our conventional rte_ring too (when
> > > > > RTE_USE_C11_MEM_MODEL enabled).
> > > > > But, so far I didn’t manage to reproduce it in reality.
> > > > > For that reason and also because it touches a critical rte_ring code-path, I
> > > put
> > > > > these changes into a separate patch. Expect all interested stakeholders to
> > > come-
> > > > > up with their comments and observations.
> > > > > Regarding performance impact – on my boxes both ring_perf_autotest
> > and
> > > > > ring_stress_autotest – show a mixed set of results: some of them
> > become
> > > few
> > > > > cycles faster, another few cycles slower.
> > > > > But so far, I didn’t notice any real degradations with that patch.
> > > > >
> > > > > Fixes: b5458e2cc483 ("ring: introduce staged ordered ring")
> > > > > Fixes: 1cc363b8ce06 ("ring: introduce HTS ring mode")
> > > > > Fixes: e6ba4731c0f3 ("ring: introduce RTS ring mode")
> > > > > Fixes: 49594a63147a ("ring/c11: relax ordering for load and store of the
> > > head")
> > > > >
> > > > > Signed-off-by: Konstantin Ananyev <konstantin.ananyev@huawei.com>
> > > > > ---
> > > > > lib/ring/rte_ring_c11_pvt.h | 27 +++++++++++++++++----------
> > > > > lib/ring/rte_ring_hts_elem_pvt.h | 6 ++++--
> > > lib/ring/rte_ring_rts_elem_pvt.h
> > > > > | 6 ++++--
> > > > > lib/ring/soring.c | 5 -----
> > > > > 4 files changed, 25 insertions(+), 19 deletions(-)
> > > > >
> > > > > diff --git a/lib/ring/rte_ring_c11_pvt.h b/lib/ring/rte_ring_c11_pvt.h
> > index
> > > > > 0845cd6dcf..6d1c46df9a 100644
> > > > > --- a/lib/ring/rte_ring_c11_pvt.h
> > > > > +++ b/lib/ring/rte_ring_c11_pvt.h
> > > > > @@ -77,20 +77,19 @@ __rte_ring_headtail_move_head(struct
> > > > > rte_ring_headtail *d,
> > > > > int success;
> > > > > unsigned int max = n;
> > > > >
> > > > > + /* Ensure the head is read before tail */
> > > > > *old_head = rte_atomic_load_explicit(&d->head,
> > > > > - rte_memory_order_relaxed);
> > > > > + rte_memory_order_acquire);
> > > > > do {
> > > > > /* Reset n to the initial burst count */
> > > > > n = max;
> > > > >
> > > > > - /* Ensure the head is read before tail */
> > > > > - rte_atomic_thread_fence(rte_memory_order_acquire);
> > > > > -
> > > > > - /* load-acquire synchronize with store-release of ht->tail
> > > > > - * in update_tail.
> > > > > + /*
> > > > > + * Read s->tail value. Note that it will be loaded after
> > > > > + * d->head load, but before CAS operation for the d->head.
> > > > > */
> > > > > stail = rte_atomic_load_explicit(&s->tail,
> > > > > - rte_memory_order_acquire);
> > > > > + rte_memory_order_relaxed);
> > > > >
> > > > > /* The subtraction is done between two unsigned 32bits value
> > > > > * (the result is always modulo 32 bits even if we have @@ -
> > > > > 112,11 +111,19 @@ __rte_ring_headtail_move_head(struct
> > > rte_ring_headtail
> > > > > *d,
> > > > > d->head = *new_head;
> > > > > success = 1;
> > > > > } else
> > > > > - /* on failure, *old_head is updated */
> > > > > + /*
> > > > > + * on failure, *old_head is updated.
> > > > > + * this CAS(ACQ_REL, ACQUIRE) serves as a hoist
> > > > > + * barrier to prevent:
> > > > > + * - OOO reads of cons tail value
> > > > > + * - OOO copy of elems from the ring
> > > > > + * Also RELEASE guarantees that latest tail value
> > > > > + * will become visible before the new head value.
> > > > > + */
> > > > > success =
> > > > > rte_atomic_compare_exchange_strong_explicit(
> > > > > &d->head, old_head, *new_head,
> > > > > - rte_memory_order_relaxed,
> > > > > - rte_memory_order_relaxed);
> > > > > + rte_memory_order_acq_rel,
> > > > > + rte_memory_order_acquire);
> > > > > } while (unlikely(success == 0));
> > > > > return n;
> > > > > }
> > > > > diff --git a/lib/ring/rte_ring_hts_elem_pvt.h
> > > b/lib/ring/rte_ring_hts_elem_pvt.h
> > > > > index c59e5f6420..cc593433b9 100644
> > > > > --- a/lib/ring/rte_ring_hts_elem_pvt.h
> > > > > +++ b/lib/ring/rte_ring_hts_elem_pvt.h
> > > > > @@ -116,13 +116,15 @@ __rte_ring_hts_move_head(struct
> > > > > rte_ring_hts_headtail *d,
> > > > > np.pos.head = op.pos.head + n;
> > > > >
> > > > > /*
> > > > > - * this CAS(ACQUIRE, ACQUIRE) serves as a hoist barrier to prevent:
> > > > > + * this CAS(ACQ_REL, ACQUIRE) serves as a hoist barrier to prevent:
> > > > > * - OOO reads of cons tail value
> > > > > * - OOO copy of elems from the ring
> > > > > + * Also RELEASE guarantees that latest tail value
> > > > > + * will become visible before the new head value.
> > > > > */
> > > > > } while (rte_atomic_compare_exchange_strong_explicit(&d->ht.raw,
> > > > > (uint64_t *)(uintptr_t)&op.raw, np.raw,
> > > > > - rte_memory_order_acquire,
> > > > > + rte_memory_order_acq_rel,
> > > > > rte_memory_order_acquire) == 0);
> > > > >
> > > > > *old_head = op.pos.head;
> > > > > diff --git a/lib/ring/rte_ring_rts_elem_pvt.h
> > > b/lib/ring/rte_ring_rts_elem_pvt.h
> > > > > index 509fa674fb..860b13cc61 100644
> > > > > --- a/lib/ring/rte_ring_rts_elem_pvt.h
> > > > > +++ b/lib/ring/rte_ring_rts_elem_pvt.h
> > > > > @@ -131,13 +131,15 @@ __rte_ring_rts_move_head(struct
> > > > > rte_ring_rts_headtail *d,
> > > > > nh.val.cnt = oh.val.cnt + 1;
> > > > >
> > > > > /*
> > > > > - * this CAS(ACQUIRE, ACQUIRE) serves as a hoist barrier to prevent:
> > > > > + * this CAS(ACQ_REL, ACQUIRE) serves as a hoist barrier to prevent:
> > > > > * - OOO reads of cons tail value
> > > > > * - OOO copy of elems to the ring
> > > > > + * Also RELEASE guarantees that latest tail value
> > > > > + * will become visible before the new head value.
> > > > > */
> > > > > } while (rte_atomic_compare_exchange_strong_explicit(&d-
> > > >head.raw,
> > > > > (uint64_t *)(uintptr_t)&oh.raw, nh.raw,
> > > > > - rte_memory_order_acquire,
> > > > > + rte_memory_order_acq_rel,
> > > > > rte_memory_order_acquire) == 0);
> > > > >
> > > > > *old_head = oh.val.pos;
> > > > > diff --git a/lib/ring/soring.c b/lib/ring/soring.c index
> > > 7bcbf35516..21a1a27e24
> > > > > 100644
> > > > > --- a/lib/ring/soring.c
> > > > > +++ b/lib/ring/soring.c
> > > > > @@ -123,8 +123,6 @@ __rte_soring_stage_finalize(struct
> > > > > soring_stage_headtail *sht, uint32_t stage,
> > > > > rte_atomic_store_explicit(&sht->tail.raw, ot.raw,
> > > > > rte_memory_order_release);
> > > > >
> > > > > - /* make sure that new tail value is visible */
> > > > > - rte_atomic_thread_fence(rte_memory_order_release);
> > > > > return i;
> > > > > }
> > > > >
> > > > > @@ -219,9 +217,6 @@ __rte_soring_update_tail(struct
> > > __rte_ring_headtail
> > > > > *rht,
> > > > > /* unsupported mode, shouldn't be here */
> > > > > RTE_ASSERT(0);
> > > > > }
> > > > > -
> > > > > - /* make sure that new tail value is visible */
> > > > > - rte_atomic_thread_fence(rte_memory_order_release);
> > > > > }
> > > > >
> > > > > static __rte_always_inline uint32_t
> > > > > --
> > > > > 2.43.0