What about the following, we shift offset by 6, as all buckets are aligned to 64, anyway, and that gives us 6 more bits so we can have 8 bit epoch counter…. ?
— Damjan > On 03.11.2021., at 19:45, Damjan Marion <dmar...@me.com> wrote: > > > > yes, i am aware of that, it is extremelly unlikely and only way i can see > this fixed is introducing epoch on the bucket level but we dont have enough > space there…. > > — > Damjan > >>> On 03.11.2021., at 19:16, Florin Coras <fcoras.li...@gmail.com> wrote: >>> >> Hi Damjan, >> >> Definitely like the scheme but the change bit might not be enough, unless >> I’m misunderstanding. For instance, two consecutive updates to a bucket >> before reader grabs b1 will hide the change. >> >> Florin >> >>> On Nov 3, 2021, at 9:36 AM, Damjan Marion via lists.fd.io >>> <dmarion=me....@lists.fd.io> wrote: >>> >>> >>> Agree with Dave on atomic ops being bad on the reader side. >>> >>> What about following schema: >>> >>> As bucket is just u64 value on the reader side we grab bucket before (b0) >>> and after (b1) search operation. >>> >>> If search finds entry, we simply do 2 checks: >>> - that b0 is equal to b1 >>> - that lock bit is not set in both of them >>> If check fails, we simply retry. >>> >>> On the writer side, we have add, remove and replace operations. >>> First 2 alter refcnt which is part of bucket. >>> To deal with replace case we introduce another bit (change bit) which is >>> flipped every time data is changed in the bucket. >>> >>> Here are possible scenarios: >>> >>> - reader grabs b0 before lock and b1 after unlock >>> - add, del - refcnt and change bit will be different between b0 and b1 >>> causing retry >>> - replace - change bit will be different between b0 and b1 causing retry >>> >>> - reader grabs b0 after lock and/or b1 before unlock >>> - lock bit will be set causing retry >>> >>> Of course, this to work properly we need to ensure proper memory ordering >>> (i.e. avoid bucket change to be visible to remote thread before kvp change). >>> >>> I crafted WIP patch to present my idea: >>> >>> https://gerrit.fd.io/r/c/vpp/+/34326 >>> >>> In this patch I get a rid of all store barriers and replaced them with more >>> lightweight: >>> >>> __atomic_store_n (ptr, val, __ATOMIC_RELEASE); >>> >>> On platforms with strong memory ordering (like x86_64) this will result >>> with just normal stores (but compiler will know that it should not reorder >>> them). >>> On platforms with weak memory ordering (like arch64) this will result in >>> special store instruction, but that one is still cheaper than full memory >>> barrier. >>> >>> Thoughts? Comments? >>> >>> Thanks, >>> >>> — >>> Damjan >>> >>> >>> >>>> On 02.11.2021., at 12:14, Dave Barach <v...@barachs.net> wrote: >>>> >>>> Dear Nick, >>>> >>>> As the code comment suggests, we tiptoe right up to the line to extract >>>> performance. Have you tried e.g. ISOLCPUS, thread priority, or some other >>>> expedients to make the required assumptions true? >>>> >>>> It’s easy enough to change the code in various ways so this use-case >>>> cannot backfire. High on the list: always make a working copy of the >>>> bucket, vs. update in place. Won’t help write performance, but it’s likely >>>> to make the pain go away. >>>> >>>> Bucket-level reader-locks would involve adding Avogadro’s number of atomic >>>> ops to the predominant case. I’m pretty sure that’s a non-starter. >>>> >>>> FWIW... Dave >>>> >>>> >>>> From: vpp-dev@lists.fd.io <vpp-dev@lists.fd.io> On Behalf Of Nick >>>> Zavaritsky >>>> Sent: Monday, November 1, 2021 12:12 PM >>>> To: vpp-dev@lists.fd.io >>>> Subject: [vpp-dev] Bihash is considered thread-safe but probably shouldn't >>>> >>>> Hello bihash experts! >>>> >>>> There's an old thread claiming that bihash lookup can produce a value=-1 >>>> under intense add/delete concurrent activity: >>>> https://lists.fd.io/g/vpp-dev/message/15606 >>>> >>>> We had a seemingly related crash recently when a lookup in >>>> snat_main.flow_hash yielded a value=-1 which was subsequently used as a >>>> destination thread index to offload to. This crash prompted me to study >>>> bihash more closely. >>>> >>>> The rest of the message is structured as follows: >>>> 1. Presenting reasons why I believe that bihash is not thread-safe. >>>> 2. Proposing a fix. >>>> >>>> 1 Bihash is probably not thread-safe >>>> >>>> The number of buckets in a hash table never changes. Every bucket has a >>>> lock bit. Updates happen via clib_bihash_add_del_inline_with_hash. The >>>> function grabs the bucket lock early on and performs update while holding >>>> the lock. Obviously this is safe, let's focus on readers. >>>> >>>> Lookups happen via clib_bihash_search_inline_with_hash / >>>> clib_bihash_search_inline_2_with_hash. The function locates the bucket and >>>> waits until the lock bit is cleared. >>>> >>>> b = BV (clib_bihash_get_bucket) (h, hash); >>>> >>>> if (PREDICT_FALSE (BV (clib_bihash_bucket_is_empty) (b))) >>>> return -1; >>>> >>>> if (PREDICT_FALSE (b->lock)) >>>> { >>>> volatile BVT (clib_bihash_bucket) * bv = b; >>>> while (bv->lock) >>>> CLIB_PAUSE (); >>>> } >>>> >>>> From this point on the function examines the data structure without ever >>>> bothering to check the lock again. Nothing prevents an updater from >>>> changing the data the reader is currently looking at, or even deallocating >>>> it right away. The only way it could work is if we could make assumptions >>>> about relative performance of lookup and update operations. Checking the >>>> lock early in lookup ensures that there's no update currently in progress. >>>> If lookup is faster than update, then no future updater will manage to >>>> progress to the point where the memory is written BEFORE the lookup was >>>> complete. Indeed, we have the following comment in >>>> clib_bihash_add_del_inline_with_hash: >>>> >>>> /* >>>> * Because reader threads are looking at live data, >>>> * we have to be extra careful. Readers do NOT hold the >>>> * bucket lock. We need to be SLOWER than a search, past the >>>> * point where readers CHECK the bucket lock. >>>> */ >>>> >>>> Unfortunately, the assumption doesn't hold. Any thread could get preempted >>>> at arbitrary time. Even if we rule out preemption, there are >>>> microarchitectural quirks (e.g. caches, branch misprediction) that could >>>> slow down lookup to the point that memory read and update will overlap. >>>> >>>> The core of lookup is the following loop. Please note that checking a key >>>> and fetching the value is not atomic, hence if we are preempted in-between >>>> the result could be bogus. >>>> >>>> for (i = 0; i < limit; i++) >>>> { >>>> if (BV (clib_bihash_key_compare) (v->kvp[i].key, key_result->key)) >>>> { >>>> *key_result = v->kvp[i]; >>>> return 0; >>>> } >>>> } >>>> >>>> Different ways the key-value pair could get updated: >>>> >>>> (1) Add using a previously empty slot: >>>> >>>> clib_memcpy_fast (&(v->kvp[i].value), >>>> &add_v->value, sizeof (add_v->value)); >>>> CLIB_MEMORY_STORE_BARRIER (); /* Make sure the value has >>>> settled */ >>>> clib_memcpy_fast (&(v->kvp[i]), &add_v->key, >>>> sizeof (add_v->key)); >>>> >>>> The key update is not atomic, reader could observe a key updated half-way. >>>> >>>> (2) Add that recycles a stale slot: >>>> >>>> clib_memcpy_fast (&(v->kvp[i]), add_v, sizeof (*add_v)); >>>> >>>> Yet again not atomic. A reader could witness (old_k, new_v) or (new_k, >>>> old_v) or even an arbitrary interleaving of chunks from old and new keys. >>>> >>>> (3) Deleting an entry: >>>> >>>> clib_memset_u8 (&(v->kvp[i]), 0xff, sizeof (*(add_v))); >>>> >>>> Not atomic. >>>> >>>> >>>> 2 A fix >>>> >>>> It's worth noting that bihash never crashes. It does yield bogus results >>>> occasionally, though. While -1 is easy to check for, the analysis shows >>>> that other bogus results are possible. In particular: >>>> >>>> 1. Value updated half-way, possible with bihash_8_16. >>>> 2. Observing a key that never existed due to a key partial update. The >>>> probability is low since the hash should map it to the same bucket. >>>> 3. Old key matched with a new value. The probability is low since lookup >>>> should get preempted at the particular spot to make it happen. >>>> >>>> Even though these anomalies are unlikely they are still possible and >>>> exploitable. >>>> >>>> Should we consider a fix? >>>> >>>> The proposal is to introduce read locks for buckets. An implementation >>>> favouring readers could be as follows: >>>> >>>> Extend clib_bihash wirh "u64 rlocks[MAX_THREADS]". Based on the thread >>>> index, each reader publishes the bucket number it is currently examining >>>> in the respective array item. Padding is introduced to avoid false sharing. >>>> >>>> The writer lock sequence would be: 1) lock bucket; 2) wait until the >>>> bucket number is not in rlocks. >>>> >>>> Reader lock sequence: 1) publish bucket number in rlocks; 2) if bucket not >>>> locked then done; 3) otherwise clear bucket number from rlocks, wait for >>>> bucket lock to be released and restart. >>>> >>>> Thoughts? >>>> >>>> >>> >>> >>> >>
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