[Tarantool-patches] [PATCH v22 2/3] qsync: order access to the limbo terms
Cyrill Gorcunov
gorcunov at gmail.com
Mon Oct 11 22:16:34 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.
Also Serge Petrenko pointed that we have somewhat similar situation
with txn_limbo_ack()[we might try to write confirm on entry while
new promote is in fly and not yet applied, so confirm might be invalid]
and txn_limbo_on_parameters_change() [where we might confirm entries
reducing quorum number while we even not a limbo owner]. Thus we need
to fix these problems as well.
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
5) txn_limbo_ack() and txn_limbo_on_parameters_change() both
respect current limbo owner via promote latch.
Part-of #6036
Signed-off-by: Cyrill Gorcunov <gorcunov at gmail.com>
---
src/box/applier.cc | 12 ++++--
src/box/box.cc | 15 ++++---
src/box/relay.cc | 11 ++---
src/box/txn.c | 2 +-
src/box/txn_limbo.c | 49 +++++++++++++++++++--
src/box/txn_limbo.h | 78 +++++++++++++++++++++++++++++++---
test/unit/snap_quorum_delay.cc | 5 ++-
7 files changed, 142 insertions(+), 30 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/relay.cc b/src/box/relay.cc
index f5852df7b..61ef1e3a5 100644
--- a/src/box/relay.cc
+++ b/src/box/relay.cc
@@ -545,15 +545,10 @@ tx_status_update(struct cmsg *msg)
ack.vclock = &status->vclock;
/*
* Let pending synchronous transactions know, which of
- * them were successfully sent to the replica. Acks are
- * collected only by the transactions originator (which is
- * the single master in 100% so far). Other instances wait
- * for master's CONFIRM message instead.
+ * them were successfully sent to the replica.
*/
- if (txn_limbo.owner_id == instance_id) {
- txn_limbo_ack(&txn_limbo, ack.source,
- vclock_get(ack.vclock, instance_id));
- }
+ txn_limbo_ack(&txn_limbo, instance_id, ack.source,
+ vclock_get(ack.vclock, instance_id));
trigger_run(&replicaset.on_ack, &ack);
static const struct cmsg_hop route[] = {
diff --git a/src/box/txn.c b/src/box/txn.c
index e7fc81683..06bb85a09 100644
--- a/src/box/txn.c
+++ b/src/box/txn.c
@@ -939,7 +939,7 @@ txn_commit(struct txn *txn)
txn_limbo_assign_local_lsn(&txn_limbo, limbo_entry,
lsn);
/* Local WAL write is a first 'ACK'. */
- txn_limbo_ack(&txn_limbo, txn_limbo.owner_id, lsn);
+ txn_limbo_ack_self(&txn_limbo, lsn);
}
if (txn_limbo_wait_complete(&txn_limbo, limbo_entry) < 0)
goto rollback;
diff --git a/src/box/txn_limbo.c b/src/box/txn_limbo.c
index 70447caaf..8f9bc11c7 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;
@@ -542,10 +543,30 @@ txn_limbo_read_demote(struct txn_limbo *limbo, int64_t lsn)
}
void
-txn_limbo_ack(struct txn_limbo *limbo, uint32_t replica_id, int64_t lsn)
+txn_limbo_ack(struct txn_limbo *limbo, uint32_t owner_id,
+ uint32_t replica_id, int64_t lsn)
{
+ /*
+ * ACKs are collected only by the transactions originator
+ * (which is the single master in 100% so far). Other instances
+ * wait for master's CONFIRM message instead.
+ *
+ * Due to cooperative multitasking there might be limbo owner
+ * migration in-fly (while writing data to journal), so for
+ * simplicity sake the test for owner is done here instead
+ * of putting this check to the callers.
+ */
+ if (!txn_limbo_is_owner(limbo, owner_id))
+ return;
+
+ /*
+ * Test for empty queue is done _after_ txn_limbo_is_owner
+ * call because we need to be sure that limbo is not been
+ * changed under our feets while we're reading it.
+ */
if (rlist_empty(&limbo->queue))
return;
+
/*
* If limbo is currently writing a rollback, it means that the whole
* queue will be rolled back. Because rollback is written only for
@@ -724,11 +745,14 @@ 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;
@@ -786,11 +810,30 @@ 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)
{
+ /*
+ * In case if we're not current leader (ie not owning the
+ * limbo) then we should not confirm anything, otherwise
+ * we could reduce quorum number and start writing CONFIRM
+ * while leader node carries own maybe bigger quorum value.
+ */
+ if (!txn_limbo_is_owner(limbo, instance_id))
+ return;
+
if (rlist_empty(&limbo->queue))
return;
+
struct txn_limbo_entry *e;
int64_t confirm_lsn = -1;
rlist_foreach_entry(e, &limbo->queue, in_queue) {
diff --git a/src/box/txn_limbo.h b/src/box/txn_limbo.h
index 53e52f676..33cacef8f 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
@@ -194,6 +199,23 @@ txn_limbo_is_empty(struct txn_limbo *limbo)
return rlist_empty(&limbo->queue);
}
+/**
+ * Test if the \a owner_id is current limbo owner.
+ */
+static inline bool
+txn_limbo_is_owner(struct txn_limbo *limbo, uint32_t owner_id)
+{
+ /*
+ * A guard needed to prevent race with in-fly promote
+ * packets which are sitting inside journal but not yet
+ * written.
+ */
+ latch_lock(&limbo->promote_latch);
+ bool v = limbo->owner_id == owner_id;
+ latch_unlock(&limbo->promote_latch);
+ return v;
+}
+
bool
txn_limbo_is_ro(struct txn_limbo *limbo);
@@ -216,9 +238,12 @@ 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 +251,14 @@ 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;
}
/**
@@ -287,7 +315,15 @@ txn_limbo_assign_lsn(struct txn_limbo *limbo, struct txn_limbo_entry *entry,
* replica with the specified ID.
*/
void
-txn_limbo_ack(struct txn_limbo *limbo, uint32_t replica_id, int64_t lsn);
+txn_limbo_ack(struct txn_limbo *limbo, uint32_t owner_id,
+ uint32_t replica_id, int64_t lsn);
+
+static inline void
+txn_limbo_ack_self(struct txn_limbo *limbo, int64_t lsn)
+{
+ return txn_limbo_ack(limbo, limbo->owner_id,
+ limbo->owner_id, lsn);
+}
/**
* Block the current fiber until the transaction in the limbo
@@ -300,7 +336,35 @@ 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)
+{
+ latch_unlock(&limbo->promote_latch);
+}
+
+/** Rollback a synchronous replication request. */
+static inline void
+txn_limbo_process_rollback(struct txn_limbo *limbo)
+{
+ 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);
diff --git a/test/unit/snap_quorum_delay.cc b/test/unit/snap_quorum_delay.cc
index 803bbbea8..d43b4cd2c 100644
--- a/test/unit/snap_quorum_delay.cc
+++ b/test/unit/snap_quorum_delay.cc
@@ -130,7 +130,7 @@ txn_process_func(va_list ap)
}
txn_limbo_assign_local_lsn(&txn_limbo, entry, fake_lsn);
- txn_limbo_ack(&txn_limbo, txn_limbo.owner_id, fake_lsn);
+ txn_limbo_ack_self(&txn_limbo, fake_lsn);
txn_limbo_wait_complete(&txn_limbo, entry);
switch (process_type) {
@@ -157,7 +157,8 @@ txn_confirm_func(va_list ap)
* inside gc_checkpoint().
*/
fiber_sleep(0);
- txn_limbo_ack(&txn_limbo, relay_id, fake_lsn);
+ txn_limbo_ack(&txn_limbo, txn_limbo.owner_id,
+ relay_id, fake_lsn);
return 0;
}
--
2.31.1
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