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Delete iterator in replication_test #295

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Delete iterator in replication_test #295

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4 changes: 3 additions & 1 deletion cloud/replication_test.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -1315,13 +1315,15 @@ TEST_F(ReplicationTest, SuperSnapshot) {
ASSERT_OK(follower->Get(ro, "k1", &val));
EXPECT_EQ(val, "v1");

auto iter = follower->NewIterator(ro, follower->DefaultColumnFamily());
auto iter = std::unique_ptr<rocksdb::Iterator>(
follower->NewIterator(ro, follower->DefaultColumnFamily()));
iter->SeekToFirst();
EXPECT_TRUE(iter->Valid());
EXPECT_EQ(iter->key(), "k1");
EXPECT_EQ(iter->value(), "v1");
iter->Next();
EXPECT_FALSE(iter->Valid());
iter.reset();

ro.snapshot = nullptr;
ASSERT_OK(follower->Get(ro, followerCF("cf1"), "cf1k1", &val));
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5 changes: 4 additions & 1 deletion db/db_impl/db_impl.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -4077,6 +4077,8 @@ Status DBImpl::GetSuperSnapshots(
return Status::InvalidArgument(
"GetSuperSnapshots only supported in RocksDB compiled with USE_RTTI=1");
#endif
InstrumentedMutexLock l(&mutex_);

if (!is_snapshot_supported_) {
return Status::InvalidArgument("Snapshot not supported");
}
Expand All @@ -4092,7 +4094,8 @@ Status DBImpl::GetSuperSnapshots(
for (auto& cf : column_families) {
auto cfh = static_cast_with_check<ColumnFamilyHandleImpl>(cf);
auto cfd = cfh->cfd();
auto sv = cfd->GetReferencedSuperVersion(this);
auto sv = cfd->GetSuperVersion();
sv->Ref();
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Not sure what the difference is here could you give me some context?

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They are equivalent, but GetReferencedSuperVersion() is meant to (and optimized to) be called outside of the lock. By adding a mutex to GetSuperSnapshots() (oops) it caused a deadlock :)

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It's using quite an interesting technique:

rocksdb-cloud/db/column_family.cc

Lines 1260 to 1277 in 43810fe

// The SuperVersion is cached in thread local storage to avoid acquiring
// mutex when SuperVersion does not change since the last use. When a new
// SuperVersion is installed, the compaction or flush thread cleans up
// cached SuperVersion in all existing thread local storage. To avoid
// acquiring mutex for this operation, we use atomic Swap() on the thread
// local pointer to guarantee exclusive access. If the thread local pointer
// is being used while a new SuperVersion is installed, the cached
// SuperVersion can become stale. In that case, the background thread would
// have swapped in kSVObsolete. We re-check the value at when returning
// SuperVersion back to thread local, with an atomic compare and swap.
// The superversion will need to be released if detected to be stale.
void* ptr = local_sv_->Swap(SuperVersion::kSVInUse);
// Invariant:
// (1) Scrape (always) installs kSVObsolete in ThreadLocal storage
// (2) the Swap above (always) installs kSVInUse, ThreadLocal storage
// should only keep kSVInUse before ReturnThreadLocalSuperVersion call
// (if no Scrape happens).
assert(ptr != SuperVersion::kSVInUse);

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Will have to read more outside the function but definitely interesting. Not quite clear to me how thread local super versions get cleaned after a swap since local_sv becomes obsolete (dump thread does it? but then wouldn't that mess up the currently used sv for the thread?)

auto ss = new SuperSnapshotImpl(cfd, sv);
snapshots_.New(ss, snapshot_seq, unix_time,
/*is_write_conflict_boundary=*/false);
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