From: Serge Petrenko via Tarantool-patches <tarantool-patches@dev.tarantool.org> To: Cyrill Gorcunov <gorcunov@gmail.com>, Vladislav Shpilevoy <v.shpilevoy@tarantool.org> Cc: tml <tarantool-patches@dev.tarantool.org> Subject: Re: [Tarantool-patches] [PATCH v27 2/3] qsync: order access to the limbo terms Date: Wed, 12 Jan 2022 17:01:00 +0300 [thread overview] Message-ID: <77b533c1-0c2f-c11d-0aa6-4109674a7025@tarantool.org> (raw) In-Reply-To: <Yd3rDf460NT+KbvJ@grain> 11.01.2022 23:39, Cyrill Gorcunov пишет: > On Mon, Jan 10, 2022 at 05:28:43PM +0300, Serge Petrenko wrote: >> Hi! Thanks for the patch! >> >> box_issue_promote() and box_issue_demote() need fine-grained locking >> anyway. >> Otherwise it’s possible that promote() is already issued, but not yet >> written to WAL, and some >> outdated request is applied by applier at that exact moment. > True. And in previous series Vlad has asked to not move in code which is > not covered by tests. So I think this is a task for the next part. Currently > we cover only the race between appliers. Let's ask Vlad, then. I feel like we should fix this now, not waiting for a full fine-grained locking patch. First of all, this is a known bug (and fine-grained locking was meant to cover everything we don't know of, just in case). Besides, simply locking issue_promote/issue_demote should be much easier than implementing the fine-grained locking patch. > >> >> You should take the lock before the WAL write, and release it only after >> txn_limbo_apply. >> >> No need to guard every limbo function there is, but we have to guard >> everything that >> writes PROMOTE/DEMOTE. > ... >> @@ -216,7 +225,7 @@ txn_limbo_last_entry(struct txn_limbo *limbo) >> * @a replica_id. >> */ >> static inline uint64_t >> -txn_limbo_replica_term(const struct txn_limbo *limbo, uint32_t >> replica_id) >> +txn_limbo_replica_term(struct txn_limbo *limbo, uint32_t replica_id) >> { >> >> >> You’ve forgot to lock the latch here, I guess. > I did it on a purpose. As you remember we've faced many problems when tried > to implement fine-grained locking inside limbo code. So I dropped this idea > eventually and I think we could start with explicit locks to cover the applier > race and then walk via small steps trying to cover the rest. Ok, then return `const ` to the function declaration, please. > >> +/** >> + * Initiate execution of a synchronous replication request. >> + */ >> +static inline void >> +txn_limbo_begin(struct txn_limbo *limbo) >> +{ >> + limbo->promote_latch_cnt++; >> + latch_lock(&limbo->promote_latch); >> >> >> I suppose you should decrease the latch_cnt right after acquiring the >> lock. >> >> Otherwise you count the sole «limbo user» together with «limbo waiters». > Yes, this will represent accumulated value. To be honest I never saw such > approach in any other code (ie increment/lock/decrement) but I think this > is fine for fibres, will do. It just looks strange to me that `synchro.queue.waiters` will be non-zero when someone simply uses the limbo. They are `waiters`, not `users` or something else. > > Cyrill -- Serge Petrenko
next prev parent reply other threads:[~2022-01-12 14:01 UTC|newest] Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-12-30 20:23 [Tarantool-patches] [PATCH v27 0/3] qsync: implement packet filtering (part 1) Cyrill Gorcunov via Tarantool-patches 2021-12-30 20:23 ` [Tarantool-patches] [PATCH v27 1/3] latch: add latch_is_locked helper Cyrill Gorcunov via Tarantool-patches 2021-12-30 20:23 ` [Tarantool-patches] [PATCH v27 2/3] qsync: order access to the limbo terms Cyrill Gorcunov via Tarantool-patches 2022-01-10 14:28 ` Serge Petrenko via Tarantool-patches 2022-01-11 20:39 ` Cyrill Gorcunov via Tarantool-patches 2022-01-12 14:01 ` Serge Petrenko via Tarantool-patches [this message] 2022-01-12 21:30 ` Vladislav Shpilevoy via Tarantool-patches 2022-01-13 10:13 ` Serge Petrenko via Tarantool-patches 2022-01-13 23:32 ` Vladislav Shpilevoy via Tarantool-patches 2022-01-14 10:20 ` Serge Petrenko via Tarantool-patches 2022-01-14 10:33 ` Cyrill Gorcunov via Tarantool-patches 2021-12-30 20:23 ` [Tarantool-patches] [PATCH v27 3/3] test: add gh-6036-qsync-order test Cyrill Gorcunov via Tarantool-patches 2022-01-10 14:29 ` Serge Petrenko via Tarantool-patches 2022-01-11 20:41 ` Cyrill Gorcunov via Tarantool-patches
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