> > > > > 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- > > <https://patchwork.dpdk.org/project/dpdk/patch/20250521111432.207936-4-> > > > > [email protected] > > <mailto:[email protected]> <mailto:20250521111432.207936-4- > > > > [email protected] > > <mailto:[email protected]>>/ > > > > > > > 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- > > <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- > > <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-> > > > > <https://patchwork.dpdk.org/project/dpdk/patch/20250521111432.207936-4- > > > <https://patchwork.dpdk.org/project/dpdk/patch/20250521111432.207936-4- > > >> > > > > > > [email protected] > > <mailto:[email protected]> > > <mailto:[email protected] > > <mailto:[email protected]>>/ > > > > > > > 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 <[email protected] > > <mailto:[email protected]> > > > > <mailto:[email protected] > > <mailto:[email protected]>>> > > > > > > > > Signed-off-by: Ola Liljedahl <[email protected] > > <mailto:[email protected]> > > > > <mailto:[email protected] <mailto:[email protected]>>> > > > > > > > > Reviewed-by: Honnappa Nagarahalli <[email protected] > > <mailto:[email protected]> > > > > <mailto:[email protected] > > <mailto:[email protected]>>> > > > > > > > > Reviewed-by: Dhruv Tripathi <[email protected] > > <mailto:[email protected]> > > > > <mailto:[email protected] <mailto:[email protected]>>> > > > > > > > > --- > > > > > > > > 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 <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). > > How do you (or the thread) know there are enough free (or used) entries? Do > > you > > assume sequentially consistent behaviour (a total order of memory accesses)? > > Otherwise, you would need to explicitly create a happens-before relation > > between threads, e.g. a consumer which made room in the ring buffer must > > synchronize-with the producer that there is now room for more elements. That > > synchronize-with edge will ensure the producer reads a fresh value of > > stail. But > > without it, how can a thread know the state of the ring buffer that is being > > manipulated by another thread? > > > > > 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). > > Single-threaded scenarios are not a problem. Do you have a multithreaded > > scenario where > > the caller expects enqueue/dequeue to always succeed? How are the threads > > involved in such > > a scenario synchronizing with each other? > > Sure, I am talking about MT scenario. > I think I already provided an example: DPDK mempool library (see below). > In brief, It works like that: > At init it allocates ring of N memory buffers and ring big enough to hold all > of them.
Sorry, I meant to say: "it allocates N memory buffers and ring big enough to hold all of them". > Then it enqueues all allocated memory buffers into the ring. > mempool_get - retrieves (dequeues) buffers from the ring. > mempool_put - puts them back (enqueues) to the ring > get() might fail (ENOMEM), while put is expected to always succeed. > > > > > > 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()?) > > I agree that using acquire/release for all prod/cons_head accesses will > > make it > > easier to > > reason about the ring buffer state. Sequential consistency (total order) is > > the > > easiest to > > reason about and often seems to be desired and expected by programmers (e.g. > > "I'll just > > add a barrier here to ensure A happens before B in this thread, now there > > is a > > total order..."). > > > > - Ola > > > > > > > > > > > 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 > > > > > > IMPORTANT NOTICE: The contents of this email and any attachments are > > confidential and may also be privileged. If you are not the intended > > recipient, > > please notify the sender immediately and do not disclose the contents to any > > other person, use it for any purpose, or store or copy the information in > > any > > medium. Thank you.
