[Tarantool-patches] [PATCH v17 2/5] qsync: order access to the limbo terms

Cyrill Gorcunov gorcunov at gmail.com
Wed Sep 22 16:05:32 MSK 2021


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 (and will
   be validating request in future in sake of split brain
   detection)
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

Since txn_limbo_process_begin() can fail we had to change all
callers to return error.

In particular box.ctrl.promote() and box.ctrl.demote() commands
now may fail. Internally they write promote or demote packet to
the WAL and process synchro queue. So to eliminate code duplication
we use new box_issue_synchro() helper which respects locking
mechanism.

Part-of #6036

Signed-off-by: Cyrill Gorcunov <gorcunov at gmail.com>
---
 src/box/applier.cc      | 16 +++++++---
 src/box/box.cc          | 71 +++++++++++++++++++----------------------
 src/box/memtx_engine.cc |  3 +-
 src/box/txn_limbo.c     | 34 ++++++++++++++++++--
 src/box/txn_limbo.h     | 49 +++++++++++++++++++++++++---
 5 files changed, 121 insertions(+), 52 deletions(-)

diff --git a/src/box/applier.cc b/src/box/applier.cc
index b981bd436..f0751b68a 100644
--- a/src/box/applier.cc
+++ b/src/box/applier.cc
@@ -458,7 +458,8 @@ applier_wait_snapshot(struct applier *applier)
 				struct synchro_request req;
 				if (xrow_decode_synchro(&row, &req) != 0)
 					diag_raise();
-				txn_limbo_process(&txn_limbo, &req);
+				if (txn_limbo_process(&txn_limbo, &req) != 0)
+					diag_raise();
 			} else if (iproto_type_is_raft_request(row.type)) {
 				struct raft_request req;
 				if (xrow_decode_raft(&row, &req, NULL) != 0)
@@ -857,7 +858,7 @@ 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_run(&txn_limbo, synchro_entry->req);
 		trigger_run(&replicaset.applier.on_wal_write, NULL);
 	}
 	fiber_wakeup(synchro_entry->owner);
@@ -873,6 +874,9 @@ apply_synchro_row(uint32_t replica_id, struct xrow_header *row)
 	if (xrow_decode_synchro(row, &req) != 0)
 		goto err;
 
+	if (txn_limbo_process_begin(&txn_limbo, &req) != 0)
+		goto err;
+
 	struct replica_cb_data rcb_data;
 	struct synchro_entry entry;
 	/*
@@ -910,12 +914,16 @@ 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 7b11d56d6..19e67b186 100644
--- a/src/box/box.cc
+++ b/src/box/box.cc
@@ -424,8 +424,7 @@ wal_stream_apply_synchro_row(struct wal_stream *stream, struct xrow_header *row)
 		say_error("couldn't decode a synchro request");
 		return -1;
 	}
-	txn_limbo_process(&txn_limbo, &syn_req);
-	return 0;
+	return txn_limbo_process(&txn_limbo, &syn_req);
 }
 
 static int
@@ -1670,48 +1669,43 @@ box_wait_limbo_acked(double timeout)
 	return wait_lsn;
 }
 
-/** Write and process a PROMOTE request. */
-static void
-box_issue_promote(uint32_t prev_leader_id, int64_t promote_lsn)
+/** Write and process PROMOTE or DEMOTE request. */
+static int
+box_issue_synchro(uint16_t type, uint32_t prev_leader_id, int64_t promote_lsn)
 {
 	struct raft *raft = box_raft();
+
+	assert(type == IPROTO_RAFT_PROMOTE ||
+	       type == IPROTO_RAFT_DEMOTE);
 	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,
-		.origin_id = instance_id,
-		.lsn = promote_lsn,
-		.term = raft->term,
+		.type		= type,
+		.replica_id	= prev_leader_id,
+		.origin_id	= instance_id,
+		.lsn		= promote_lsn,
+		.term		= raft->term,
 	};
-	txn_limbo_process(&txn_limbo, &req);
+
+	if (txn_limbo_process_begin(&txn_limbo, &req) != 0)
+		return -1;
+
+	if (type == IPROTO_RAFT_PROMOTE)
+		txn_limbo_write_promote(&txn_limbo, req.lsn, req.term);
+	else
+		txn_limbo_write_demote(&txn_limbo, req.lsn, req.term);
+
+	txn_limbo_process_run(&txn_limbo, &req);
 	assert(txn_limbo_is_empty(&txn_limbo));
+
+	txn_limbo_process_commit(&txn_limbo);
+	return 0;
 }
 
 /** A guard to block multiple simultaneous promote()/demote() invocations. */
 static bool is_in_box_promote = false;
 
-/** Write and process a DEMOTE request. */
-static void
-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,
-		.origin_id = instance_id,
-		.lsn = promote_lsn,
-		.term = box_raft()->term,
-	};
-	txn_limbo_process(&txn_limbo, &req);
-	assert(txn_limbo_is_empty(&txn_limbo));
-}
-
 int
 box_promote_qsync(void)
 {
@@ -1732,8 +1726,8 @@ box_promote_qsync(void)
 		diag_set(ClientError, ER_NOT_LEADER, raft->leader);
 		return -1;
 	}
-	box_issue_promote(txn_limbo.owner_id, wait_lsn);
-	return 0;
+	return box_issue_synchro(IPROTO_RAFT_PROMOTE,
+				 txn_limbo.owner_id, wait_lsn);
 }
 
 int
@@ -1789,9 +1783,8 @@ box_promote(void)
 	if (wait_lsn < 0)
 		return -1;
 
-	box_issue_promote(txn_limbo.owner_id, wait_lsn);
-
-	return 0;
+	return box_issue_synchro(IPROTO_RAFT_PROMOTE,
+				 txn_limbo.owner_id, wait_lsn);
 }
 
 int
@@ -1826,8 +1819,8 @@ box_demote(void)
 	int64_t wait_lsn = box_wait_limbo_acked(replication_synchro_timeout);
 	if (wait_lsn < 0)
 		return -1;
-	box_issue_demote(txn_limbo.owner_id, wait_lsn);
-	return 0;
+	return box_issue_synchro(IPROTO_RAFT_DEMOTE,
+				 txn_limbo.owner_id, wait_lsn);
 }
 
 int
diff --git a/src/box/memtx_engine.cc b/src/box/memtx_engine.cc
index de918c335..09f1d671c 100644
--- a/src/box/memtx_engine.cc
+++ b/src/box/memtx_engine.cc
@@ -238,8 +238,7 @@ memtx_engine_recover_synchro(const struct xrow_header *row)
 	 * because all its rows have a zero replica_id.
 	 */
 	req.origin_id = req.replica_id;
-	txn_limbo_process(&txn_limbo, &req);
-	return 0;
+	return txn_limbo_process(&txn_limbo, &req);
 }
 
 static int
diff --git a/src/box/txn_limbo.c b/src/box/txn_limbo.c
index 70447caaf..eb9aa7780 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_run(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 +790,32 @@ txn_limbo_process(struct txn_limbo *limbo, const struct synchro_request *req)
 	return;
 }
 
+int
+txn_limbo_process_begin(struct txn_limbo *limbo,
+			const struct synchro_request *req)
+{
+	latch_lock(&limbo->promote_latch);
+	/*
+	 * FIXME: For now we take a lock only but idea
+	 * is to verify incoming requests to detect split
+	 * brain situation. Thus we need to change the code
+	 * semantics in advance.
+	 */
+	(void)req;
+	return 0;
+}
+
+int
+txn_limbo_process(struct txn_limbo *limbo,
+		  const struct synchro_request *req)
+{
+	if (txn_limbo_process_begin(limbo, req) != 0)
+		return -1;
+	txn_limbo_process_run(limbo, req);
+	txn_limbo_process_commit(limbo);
+	return 0;
+}
+
 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..afe9b429f 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;
 }
 
 /**
@@ -300,8 +311,36 @@ 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);
 
+/**
+ * Initiate execution of a synchronous replication request.
+ */
+int
+txn_limbo_process_begin(struct txn_limbo *limbo,
+			const struct synchro_request *req);
+
+/** 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);
+}
+
 /** Execute a synchronous replication request. */
 void
+txn_limbo_process_run(struct txn_limbo *limbo,
+		      const struct synchro_request *req);
+
+/** Process a synchronous replication request. */
+int
 txn_limbo_process(struct txn_limbo *limbo, const struct synchro_request *req);
 
 /**
-- 
2.31.1



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