Re: [Tarantool-patches] [PATCH v27 2/3] qsync: order access to the limbo terms

[Serge Petrenko via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

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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outdated request is applied by applier at that exact moment.
 
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.
  
>Четверг, 30 декабря 2021, 23:24 +03:00 от Cyrill Gorcunov < gorcunov@gmail.com >:
> 
>Limbo terms tracking is shared between appliers and when
>one of appliers is waiting for write to complete inside
>journal_write() routine, an other may need to access read
>term value to figure out if promote request is valid to
>apply. Due to cooperative multitasking access to the terms
>is not consistent so we need to be sure that other fibers
>read up to date terms (ie written to the WAL).
>
>For this sake we use a latching mechanism, when one fiber
>takes a lock for updating other readers are waiting until
>the operation is complete.
>
>For example here is a call graph of two appliers
>
>applier 1
>---------
>applier_apply_tx
>  (promote term = 3
>   current max term = 2)
>  applier_synchro_filter_tx
>  apply_synchro_row
>    journal_write
>      (sleeping)
>
>at this moment another applier comes in with obsolete
>data and term 2
>
>                              applier 2
>                              ---------
>                              applier_apply_tx
>                                (term 2)
>                                applier_synchro_filter_tx
>                                  txn_limbo_is_replica_outdated -> false
>                                journal_write (sleep)
>
>applier 1
>---------
>journal wakes up
>  apply_synchro_row_cb
>    set max term to 3
>
>So the applier 2 didn't notice that term 3 is already seen
>and wrote obsolete data. With locking the applier 2 will
>wait until applier 1 has finished its write.
>
>We introduce the following helpers:
>
>1) txn_limbo_begin: which takes a lock
>2) txn_limbo_commit and txn_limbo_rollback which simply release
>   the lock but have different names for better semantics
>3) txn_limbo_process is a general function which uses x_begin
>   and x_commit helper internally
>4) txn_limbo_apply to do a real job over processing the
>   request, it implies that txn_limbo_begin been called
>
>Testing such in-flight condition won't be easy so we introduce
>"box.info.synchro.queue.waiters" field which represent current
>number of fibers waiting for limbo to finish request processing.
>
>@TarantoolBot document
>Title: synchronous replication changes
>
>`box.info.synchro.queue` gets a new field: `waiters`. It represents
>current number of fibers waiting the synchronous transaction processing
>to complete.
>
>Part-of #6036
>
>Signed-off-by: Cyrill Gorcunov < gorcunov@gmail.com >
>---
> src/box/applier.cc | 12 ++++++++---
> src/box/lua/info.c | 4 +++-
> src/box/txn_limbo.c | 18 ++++++++++++++--
> src/box/txn_limbo.h | 52 ++++++++++++++++++++++++++++++++++++++++-----
> 4 files changed, 75 insertions(+), 11 deletions(-)
> 
 
…
 
>diff --git a/src/box/txn_limbo.h b/src/box/txn_limbo.h
>index 53e52f676..42d572595 100644
>--- a/src/box/txn_limbo.h
>+++ b/src/box/txn_limbo.h
 
…
 
>@@ -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.
 
>  return vclock_get(&limbo->promote_term_map, replica_id);
> }
>@@ -226,11 +235,14 @@ txn_limbo_replica_term(const struct txn_limbo *limbo, uint32_t replica_id)
>  * data from it. The check is only valid when elections are enabled.
>  */
> static inline bool
>-txn_limbo_is_replica_outdated(const struct txn_limbo *limbo,
>+txn_limbo_is_replica_outdated(struct txn_limbo *limbo,
>  uint32_t replica_id)
> {
>- return txn_limbo_replica_term(limbo, replica_id) <
>- limbo->promote_greatest_term;
>+ latch_lock(&limbo->promote_latch);
>+ uint64_t v = vclock_get(&limbo->promote_term_map, replica_id);
>+ bool res = v < limbo->promote_greatest_term;
>+ latch_unlock(&limbo->promote_latch);
>+ return res;
> }
> 
> /**
>@@ -300,7 +312,37 @@ txn_limbo_ack(struct txn_limbo *limbo, uint32_t replica_id, int64_t lsn);
> int
> txn_limbo_wait_complete(struct txn_limbo *limbo, struct txn_limbo_entry *entry);
> 
>-/** Execute a synchronous replication request. */
>+/**
>+ * 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».
 
>+}
>+
>+/** Commit a synchronous replication request. */
>+static inline void
>+txn_limbo_commit(struct txn_limbo *limbo)
>+{
>+ latch_unlock(&limbo->promote_latch);
>+ limbo->promote_latch_cnt--;
>+}
>+
>+/** Rollback a synchronous replication request. */
>+static inline void
>+txn_limbo_rollback(struct txn_limbo *limbo)
>+{
>+ latch_unlock(&limbo->promote_latch);
 
If you don’t want to decrease the counter right after latch_lock(), you should decrease it
here, as well as in txn_limbo_commit().
 
>+}
>+
>+/** Apply a synchronous replication request after processing stage. */
>+void
>+txn_limbo_apply(struct txn_limbo *limbo,
>+ const struct synchro_request *req);
>+
>+/** Process a synchronous replication request. */
> void
> txn_limbo_process(struct txn_limbo *limbo, const struct synchro_request *req);
> 
>--
>2.31.1
 

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