[PATCH 1/1] ring: safe partial ordering for head/tail update

[Konstantin Ananyev]

> >
> > To avoid information loss I combined reply to two Wathsala replies into one.
> >
> >
> > > > > The function __rte_ring_headtail_move_head() assumes that the
> > > > > barrier
> > > > (fence) between the load of the head and the load-acquire of the
> > > > > opposing tail guarantees the following: if a first thread reads
> > > > > tail
> > > > > and then writes head and a second thread reads the new value of
> > > > > head
> > > > > and then reads tail, then it should observe the same (or a later)
> > > > > value of tail.
> > > > >
> > > > > This assumption is incorrect under the C11 memory model. If the
> > > > > barrier
> > > > > (fence) is intended to establish a total ordering of ring
> > > > > operations,
> > > > > it fails to do so. Instead, the current implementation only
> > > > > enforces a
> > > > > partial ordering, which can lead to unsafe interleavings. In
> > > > > particular,
> > > > > some partial orders can cause underflows in free slot or available
> > > > > element computations, potentially resulting in data corruption.
> > > >
> > > > Hmm... sounds exactly like the problem from the patch we discussed
> > > > earlier that year:
> > > > https://patchwork.dpdk.org/project/dpdk/patch/20250521111432.207936-4-
> konstantin.ananyev at huawei.com <mailto:20250521111432.207936-4-
> konstantin.ananyev at huawei.com>/
> > > > In two words:
> > > > "... 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."
> > > > Is that the one?
> >
> >
> > > Yes, the behavior occurs under RCpc (LDAPR) but not under RCsc (LDAR),
> > > which is why we didn’t catch it earlier. A fuller explanation, with
> > > Herd7 simulations, is in the blog post linked in the cover letter.
> > >
> > > https://community.arm.com/arm-community-blogs/b/architectures-and-
> processors-blog/posts/when-a-barrier-does-not-block-the-pitfalls-of-partial-order
> <https://community.arm.com/arm-community-blogs/b/architectures-and-
> processors-blog/posts/when-a-barrier-does-not-block-the-pitfalls-of-partial-order>
> >
> >
> > I see, so now it is reproducible with core rte_ring on real HW.
> >
> >
> > > >
> > > > > The issue manifests when a CPU first acts as a producer and later
> > > > > as a
> > > > > consumer. In this scenario, the barrier assumption may fail when
> > > > > another
> > > > > core takes the consumer role. A Herd7 litmus test in C11 can
> > > > > demonstrate
> > > > > this violation. The problem has not been widely observed so far
> > > > > because:
> > > > > (a) on strong memory models (e.g., x86-64) the assumption holds,
> > > > > and
> > > > > (b) on relaxed models with RCsc semantics the ordering is still
> > > > > strong
> > > > > enough to prevent hazards.
> > > > > The problem becomes visible only on weaker models, when load-
> > > > > acquire is
> > > > > implemented with RCpc semantics (e.g. some AArch64 CPUs which
> > > > > support
> > > > > the LDAPR and LDAPUR instructions).
> > > > >
> > > > > Three possible solutions exist:
> > > > > 1. Strengthen ordering by upgrading release/acquire semantics to
> > > > > sequential consistency. This requires using seq-cst for
> > > > > stores,
> > > > > loads, and CAS operations. However, this approach introduces a
> > > > > significant performance penalty on relaxed-memory
> > > > > architectures.
> > > > >
> > > > > 2. Establish a safe partial order by enforcing a pair-wise
> > > > > happens-before relationship between thread of same role by
> > > > > changing
> > > > > the CAS and the preceding load of the head by converting them
> > > > > to
> > > > > release and acquire respectively. This approach makes the
> > > > > original
> > > > > barrier assumption unnecessary and allows its removal.
> > > >
> > > > For the sake of clarity, can you outline what would be exact code
> > > > changes for
> > > > approach #2? Same as in that patch:
> > > > https://patchwork.dpdk.org/project/dpdk/patch/20250521111432.207936-4-
> <https://patchwork.dpdk.org/project/dpdk/patch/20250521111432.207936-4->
> > > konstantin.ananyev at huawei.com <mailto:konstantin.ananyev at huawei.com>/
> > > > Or something different?
> > >
> > > Sorry, I missed the later half you your comment before.
> > > Yes, you have proposed the same solution there.
> >
> >
> > Ok, thanks for confirmation.
> >
> >
> > > >
> > > >
> > > > > 3. Retain partial ordering but ensure only safe partial orders
> > > > > are
> > > > > committed. This can be done by detecting underflow conditions
> > > > > (producer < consumer) and quashing the update in such cases.
> > > > > This approach makes the original barrier assumption
> > > > > unnecessary
> > > > > and allows its removal.
> > > >
> > > > > This patch implements solution (3) for performance reasons.
> > > > >
> > > > > Signed-off-by: Wathsala Vithanage <wathsala.vithanage at arm.com
> <mailto:wathsala.vithanage at arm.com>>
> > > > > Signed-off-by: Ola Liljedahl <ola.liljedahl at arm.com
> <mailto:ola.liljedahl at arm.com>>
> > > > > Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli at arm.com
> <mailto:honnappa.nagarahalli at arm.com>>
> > > > > Reviewed-by: Dhruv Tripathi <dhruv.tripathi at arm.com
> <mailto:dhruv.tripathi at arm.com>>
> > > > > ---
> > > > > lib/ring/rte_ring_c11_pvt.h | 10 +++++++---
> > > > > 1 file changed, 7 insertions(+), 3 deletions(-)
> > > > >
> > > > > diff --git a/lib/ring/rte_ring_c11_pvt.h
> > > > > b/lib/ring/rte_ring_c11_pvt.h
> > > > > index b9388af0da..e5ac1f6b9e 100644
> > > > > --- a/lib/ring/rte_ring_c11_pvt.h
> > > > > +++ b/lib/ring/rte_ring_c11_pvt.h
> > > > > @@ -83,9 +83,6 @@ __rte_ring_headtail_move_head(struct
> > > > > rte_ring_headtail
> > > > > *d,
> > > > > /* 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.
> > > > > */
> > > >
> > > > But then cons.head can be read a before prod.tail (and visa-versa),
> > > > right?
> > >
> > > Right, we let it happen but eliminate any resulting states that are
> > > semantically incorrect at the end.
> >
> >
> > Two comments here:
> > 1) I think it is probably safer to do the check like that:
> > If (*entries > ring->capacity) ...
> Yes, this might be another way of handling underflow situations. We could study
> this.
> 
> I have used the check for negative without problems in my ring buffer
> implementations
> https://github.com/ARM-software/progress64/blob/master/src/p64_ringbuf.c
> but can't say that has been battle-tested.

My thought was about the case (probably hypothetical) when the difference
between stale tail and head will be bigger then 2^31 + 1. 
 
> > 2) My concern that without forcing a proper read ordering
> > (cons.head first then prod.tail) we re-introduce a window for all sorts of
> > ABA-like problems.
> Head and tail indexes are monotonically increasing so I don't see a risk for ABA-like
> problems.

I understand that, but with current CPU speeds it can take rte_ring just few seconds to
wrap around head/tail values. If user doing something really fancy - like using rte_ring ZC API
(i.e. just moving head/tail without reading actual objects) that can probably happen even
faster (less than a second?).
Are we sure that the stale tail value will never persist that long?
Let say user calling move_head() in a loop till it succeeds?  

> Indeed, adding a monotonically increasing tag to pointers is the common way of
> avoiding ABA
> problems in lock-free designs.

Yep, using 64-bit values for head/tail counters will help to avoid these concerns.
But it will probably break HTS/RTS modes, plus it is an ABI change for sure.
 
Actually after another thought, I have one more concern here:

+               /*
+                * Ensure the entries calculation was not based on a stale
+                * and unsafe stail observation that causes underflow.
+                */
+               if ((int)*entries < 0)
+                       *entries = 0;
+
 
With that change, it might return not-valid information back to the user
about number of free/occupied entries in the ring.
Plus rte_ring_enqueue() now might fail even when there are enough free entries
in the ring (same for dequeue).
That looks like a change in our public API behavior that might break many things.
There are quite few places when caller expects enqueue/dequeue
operation to always succeed (let say there always should be enough free space in the ring).
For example: rte_mempool works like that.
I am pretty sure there are quite few other places like that inside DPDK,
not to mention third-party code.

Considering all of the above, I am actually more in favor
to combine approaches #2 and #3 for the final patch:  
establish a safe partial order (#2) and keep the check from #3 (should it become an assert()/verify()?) 

Another thing to note: whatever final approach we choose -
we need to make sure that the problem is addressed across all other
rte_ring flavors/modes too (generic implementation, rts/hts mode, soring).

Konstantin