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From: Serge Petrenko via Tarantool-patches <tarantool-patches@dev.tarantool.org>
To: Cyrill Gorcunov <gorcunov@gmail.com>,
	tml <tarantool-patches@dev.tarantool.org>
Cc: Vladislav Shpilevoy <v.shpilevoy@tarantool.org>
Subject: Re: [Tarantool-patches] [PATCH v19 2/3] qsync: order access to the limbo terms
Date: Fri, 1 Oct 2021 15:14:52 +0300
Message-ID: <0c64d172-4fa8-29ec-7845-ff772738c09a@tarantool.org> (raw)
In-Reply-To: <20210930094445.316694-3-gorcunov@gmail.com>



30.09.2021 12:44, Cyrill Gorcunov пишет:
> 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_process_begin: which takes a lock
> 2) txn_limbo_process_commit and txn_limbo_process_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_process_core to do a real job over processing the
>     request, it implies that txn_limbo_process_begin been called
>
> Part-of #6036
>
> Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
> ---
>   src/box/applier.cc  | 12 +++++++---
>   src/box/box.cc      | 15 ++++++++-----
>   src/box/txn_limbo.c | 17 +++++++++++++--
>   src/box/txn_limbo.h | 53 ++++++++++++++++++++++++++++++++++++++++-----
>   4 files changed, 80 insertions(+), 17 deletions(-)
>
> diff --git a/src/box/applier.cc b/src/box/applier.cc
> index b981bd436..46c36e259 100644
> --- a/src/box/applier.cc
> +++ b/src/box/applier.cc
> @@ -857,7 +857,7 @@ apply_synchro_row_cb(struct journal_entry *entry)
>   		applier_rollback_by_wal_io(entry->res);
>   	} else {
>   		replica_txn_wal_write_cb(synchro_entry->rcb);
> -		txn_limbo_process(&txn_limbo, synchro_entry->req);
> +		txn_limbo_process_core(&txn_limbo, synchro_entry->req);
>   		trigger_run(&replicaset.applier.on_wal_write, NULL);
>   	}
>   	fiber_wakeup(synchro_entry->owner);
> @@ -873,6 +873,8 @@ apply_synchro_row(uint32_t replica_id, struct xrow_header *row)
>   	if (xrow_decode_synchro(row, &req) != 0)
>   		goto err;
>   
> +	txn_limbo_process_begin(&txn_limbo);
> +
>   	struct replica_cb_data rcb_data;
>   	struct synchro_entry entry;
>   	/*
> @@ -910,12 +912,16 @@ apply_synchro_row(uint32_t replica_id, struct xrow_header *row)
>   	 * transactions side, including the async ones.
>   	 */
>   	if (journal_write(&entry.base) != 0)
> -		goto err;
> +		goto err_rollback;
>   	if (entry.base.res < 0) {
>   		diag_set_journal_res(entry.base.res);
> -		goto err;
> +		goto err_rollback;
>   	}
> +	txn_limbo_process_commit(&txn_limbo);
>   	return 0;
> +
> +err_rollback:
> +	txn_limbo_process_rollback(&txn_limbo);
>   err:
>   	diag_log();
>   	return -1;
> diff --git a/src/box/box.cc b/src/box/box.cc
> index e082e1a3d..6a9be745a 100644
> --- a/src/box/box.cc
> +++ b/src/box/box.cc
> @@ -1677,8 +1677,6 @@ box_issue_promote(uint32_t prev_leader_id, int64_t promote_lsn)
>   	struct raft *raft = box_raft();
>   	assert(raft->volatile_term == raft->term);
>   	assert(promote_lsn >= 0);
> -	txn_limbo_write_promote(&txn_limbo, promote_lsn,
> -				raft->term);
>   	struct synchro_request req = {
>   		.type = IPROTO_RAFT_PROMOTE,
>   		.replica_id = prev_leader_id,
> @@ -1686,8 +1684,11 @@ box_issue_promote(uint32_t prev_leader_id, int64_t promote_lsn)
>   		.lsn = promote_lsn,
>   		.term = raft->term,
>   	};
> -	txn_limbo_process(&txn_limbo, &req);
> +	txn_limbo_process_begin(&txn_limbo);
> +	txn_limbo_write_promote(&txn_limbo, req.lsn, req.term);
> +	txn_limbo_process_core(&txn_limbo, &req);
>   	assert(txn_limbo_is_empty(&txn_limbo));
> +	txn_limbo_process_commit(&txn_limbo);
>   }
>   
>   /** A guard to block multiple simultaneous promote()/demote() invocations. */
> @@ -1699,8 +1700,6 @@ box_issue_demote(uint32_t prev_leader_id, int64_t promote_lsn)
>   {
>   	assert(box_raft()->volatile_term == box_raft()->term);
>   	assert(promote_lsn >= 0);
> -	txn_limbo_write_demote(&txn_limbo, promote_lsn,
> -				box_raft()->term);
>   	struct synchro_request req = {
>   		.type = IPROTO_RAFT_DEMOTE,
>   		.replica_id = prev_leader_id,
> @@ -1708,8 +1707,12 @@ box_issue_demote(uint32_t prev_leader_id, int64_t promote_lsn)
>   		.lsn = promote_lsn,
>   		.term = box_raft()->term,
>   	};
> -	txn_limbo_process(&txn_limbo, &req);
> +	txn_limbo_process_begin(&txn_limbo);
> +	txn_limbo_write_demote(&txn_limbo, promote_lsn,
> +				box_raft()->term);
> +	txn_limbo_process_core(&txn_limbo, &req);
>   	assert(txn_limbo_is_empty(&txn_limbo));
> +	txn_limbo_process_commit(&txn_limbo);
>   }
>   
>   int
> diff --git a/src/box/txn_limbo.c b/src/box/txn_limbo.c
> index 70447caaf..855c98c98 100644
> --- a/src/box/txn_limbo.c
> +++ b/src/box/txn_limbo.c
> @@ -47,6 +47,7 @@ txn_limbo_create(struct txn_limbo *limbo)
>   	vclock_create(&limbo->vclock);
>   	vclock_create(&limbo->promote_term_map);
>   	limbo->promote_greatest_term = 0;
> +	latch_create(&limbo->promote_latch);
>   	limbo->confirmed_lsn = 0;
>   	limbo->rollback_count = 0;
>   	limbo->is_in_rollback = false;
> @@ -724,11 +725,14 @@ txn_limbo_wait_empty(struct txn_limbo *limbo, double timeout)
>   }
>   
>   void
> -txn_limbo_process(struct txn_limbo *limbo, const struct synchro_request *req)
> +txn_limbo_process_core(struct txn_limbo *limbo,
> +		       const struct synchro_request *req)
>   {
> +	assert(latch_is_locked(&limbo->promote_latch));
> +
>   	uint64_t term = req->term;
>   	uint32_t origin = req->origin_id;
> -	if (txn_limbo_replica_term(limbo, origin) < term) {
> +	if (vclock_get(&limbo->promote_term_map, origin) < (int64_t)term) {
>   		vclock_follow(&limbo->promote_term_map, origin, term);
>   		if (term > limbo->promote_greatest_term)
>   			limbo->promote_greatest_term = term;
> @@ -786,6 +790,15 @@ txn_limbo_process(struct txn_limbo *limbo, const struct synchro_request *req)
>   	return;
>   }
>   
> +void
> +txn_limbo_process(struct txn_limbo *limbo,
> +		  const struct synchro_request *req)
> +{
> +	txn_limbo_process_begin(limbo);
> +	txn_limbo_process_core(limbo, req);
> +	txn_limbo_process_commit(limbo);
> +}
> +
>   void
>   txn_limbo_on_parameters_change(struct txn_limbo *limbo)
>   {
> diff --git a/src/box/txn_limbo.h b/src/box/txn_limbo.h
> index 53e52f676..fdb214711 100644
> --- a/src/box/txn_limbo.h
> +++ b/src/box/txn_limbo.h
> @@ -31,6 +31,7 @@
>    */
>   #include "small/rlist.h"
>   #include "vclock/vclock.h"
> +#include "latch.h"
>   
>   #include <stdint.h>
>   
> @@ -147,6 +148,10 @@ struct txn_limbo {
>   	 * limbo and raft are in sync and the terms are the same.
>   	 */
>   	uint64_t promote_greatest_term;
> +	/**
> +	 * To order access to the promote data.
> +	 */
> +	struct latch promote_latch;
>   	/**
>   	 * Maximal LSN gathered quorum and either already confirmed in WAL, or
>   	 * whose confirmation is in progress right now. Any attempt to confirm
> @@ -216,9 +221,12 @@ 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)
>   {
> -	return vclock_get(&limbo->promote_term_map, replica_id);
> +	latch_lock(&limbo->promote_latch);
> +	uint64_t v = vclock_get(&limbo->promote_term_map, replica_id);
> +	latch_unlock(&limbo->promote_latch);
> +	return v;
>   }
>   
>   /**
> @@ -226,11 +234,14 @@ 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 +311,37 @@ 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_process_begin(struct txn_limbo *limbo)
> +{
> +	latch_lock(&limbo->promote_latch);
> +}
> +
> +/** Commit a synchronous replication request. */
> +static inline void
> +txn_limbo_process_commit(struct txn_limbo *limbo)
> +{
> +	assert(latch_is_locked(&limbo->promote_latch));
> +	latch_unlock(&limbo->promote_latch);
> +}
> +
> +/** Rollback a synchronous replication request. */
> +static inline void
> +txn_limbo_process_rollback(struct txn_limbo *limbo)
> +{
> +	assert(latch_is_locked(&limbo->promote_latch));
> +	latch_unlock(&limbo->promote_latch);
> +}
> +
> +/** Core of processing synchronous replication request. */
> +void
> +txn_limbo_process_core(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);

Thanks for the patch!

Mostly ok with one question:

What about txn_limbo_write_confirm/txn_limbo_read_confirm pairs issued
inside txn_limbo_ack() and txn_limbo_on_parameters_change() ?

Shouldn't they take the latch as well? I mean, txn_limbo_ack() and
txn_limbo_on_parameters_change() as a whole.
>   

-- 
Serge Petrenko


  reply	other threads:[~2021-10-01 12:14 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-30  9:44 [Tarantool-patches] [PATCH v19 0/3] qsync: implement packet filtering (part 1) Cyrill Gorcunov via Tarantool-patches
2021-09-30  9:44 ` [Tarantool-patches] [PATCH v19 1/3] latch: add latch_is_locked helper Cyrill Gorcunov via Tarantool-patches
2021-09-30  9:44 ` [Tarantool-patches] [PATCH v19 2/3] qsync: order access to the limbo terms Cyrill Gorcunov via Tarantool-patches
2021-10-01 12:14   ` Serge Petrenko via Tarantool-patches [this message]
2021-10-01 12:31     ` Cyrill Gorcunov via Tarantool-patches
2021-10-01 12:37       ` Serge Petrenko via Tarantool-patches
2021-10-04 21:53         ` Cyrill Gorcunov via Tarantool-patches
2021-10-05 13:25           ` Serge Petrenko via Tarantool-patches
2021-10-05 21:52             ` Cyrill Gorcunov via Tarantool-patches
2021-09-30  9:44 ` [Tarantool-patches] [PATCH v19 3/3] test: add gh-6036-qsync-order test Cyrill Gorcunov via Tarantool-patches
2021-10-01 12:30   ` Serge Petrenko via Tarantool-patches
2021-10-04 21:16     ` Cyrill Gorcunov via Tarantool-patches
2021-10-05 13:55       ` Serge Petrenko via Tarantool-patches
2021-10-05 22:26         ` Cyrill Gorcunov via Tarantool-patches
2021-10-05 22:32           ` Cyrill Gorcunov via Tarantool-patches
2021-10-06  7:06             ` Cyrill Gorcunov via Tarantool-patches

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