[Tarantool-patches] [PATCH v27 2/3] qsync: order access to the limbo terms
Cyrill Gorcunov
gorcunov at gmail.com
Tue Jan 11 23:39:41 MSK 2022
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 outdated request is applied by applier at that exact moment.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.
>
> 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.
> +/**
> + * 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.
Cyrill
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