NextGraph
/
rocksdb
1
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity

Fix a bug in hash linked list (#10401)

Browse Source 
Summary:
In hash linked list, with a bucket of only one record, following sequence can cause users to temporarily miss a record:

Thread 1: Fetch the structure bucket x points too, which would be a Node n1 for a key, with next pointer to be null
Thread 2: Insert a key to bucket x that is larger than the existing key. This will make n1->next points to a new node n2, and update bucket x to point to n1.
Thread 1: see n1->next is not null, so it thinks it is a header of linked list and ignore the key of n1.

Fix it by refetch structure that bucket x points to when it sees n1->next is not null. This should work because if n1->next is not null, bucket x should already point to a linked list or skip list header.

A related change is to revert th order of testing for linked list and skip list. This is because after refetching the bucket, it might end up with a skip list, rather than linked list.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10401

Test Plan: Run existing tests and make sure at least it doesn't regress.

Reviewed By: jay-zhuang

Differential Revision: D38064471

fbshipit-source-id: 142bb85e1546c803f47e3357aef3e76debccd8df
main
sdong 2 years ago committed by Facebook GitHub Bot
parent 6a160e1fec
commit 4e00748098
2 changed files with 101 additions and 78 deletions
Show all changes Ignore whitespace when comparing lines Ignore changes in amount of whitespace Ignore changes in whitespace at EOL
Show Stats Download Patch File Download Diff File
  1. 1
      HISTORY.md
  2. 178
      memtable/hash_linklist_rep.cc

1
HISTORY.md
Unescape View File

@ -13,6 +13,7 @@
* Fix a bug where `GenericRateLimiter` could revert the bandwidth set dynamically using `SetBytesPerSecond()` when a user configures a structure enclosing it, e.g., using `GetOptionsFromString()` to configure an `Options` that references an existing `RateLimiter` object.
* Fix race conditions in `GenericRateLimiter`.
* Fix a bug in `FIFOCompactionPicker::PickTTLCompaction` where total_size calculating might cause underflow
* Fix data race bug in hash linked list memtable. With this bug, read request might temporarily miss an old record in the memtable in a race condition to the hash bucket.
## 7.5.0 (07/15/2022)
### New Features

178
memtable/hash_linklist_rep.cc
Unescape View File

@ -209,10 +209,16 @@ class HashLinkListRep : public MemTableRep {
bool LinkListContains(Node* head, const Slice& key) const;
SkipListBucketHeader* GetSkipListBucketHeader(Pointer* first_next_pointer)
const;
bool IsEmptyBucket(Pointer& bucket_pointer) const {
return bucket_pointer.load(std::memory_order_acquire) == nullptr;
}
// Precondition: GetLinkListFirstNode() must have been called first and return
// null so that it must be a skip list bucket
SkipListBucketHeader* GetSkipListBucketHeader(Pointer& bucket_pointer) const;
Node* GetLinkListFirstNode(Pointer* first_next_pointer) const;
// Returning nullptr indicates it is a skip list bucket.
Node* GetLinkListFirstNode(Pointer& bucket_pointer) const;
Slice GetPrefix(const Slice& internal_key) const {
return transform_->Transform(ExtractUserKey(internal_key));
@ -222,11 +228,9 @@ class HashLinkListRep : public MemTableRep {
return GetSliceRangedNPHash(slice, bucket_size_);
}
Pointer* GetBucket(size_t i) const {
return static_cast<Pointer*>(buckets_[i].load(std::memory_order_acquire));
}
Pointer& GetBucket(size_t i) const { return buckets_[i]; }
Pointer* GetBucket(const Slice& slice) const {
Pointer& GetBucket(const Slice& slice) const {
return GetBucket(GetHash(slice));
}
@ -414,30 +418,39 @@ class HashLinkListRep : public MemTableRep {
// Advance to the first entry with a key >= target
void Seek(const Slice& k, const char* memtable_key) override {
auto transformed = memtable_rep_.GetPrefix(k);
auto* bucket = memtable_rep_.GetBucket(transformed);
SkipListBucketHeader* skip_list_header =
memtable_rep_.GetSkipListBucketHeader(bucket);
if (skip_list_header != nullptr) {
// The bucket is organized as a skip list
if (!skip_list_iter_) {
skip_list_iter_.reset(
new MemtableSkipList::Iterator(&skip_list_header->skip_list));
} else {
skip_list_iter_->SetList(&skip_list_header->skip_list);
}
if (memtable_key != nullptr) {
skip_list_iter_->Seek(memtable_key);
Pointer& bucket = memtable_rep_.GetBucket(transformed);
if (memtable_rep_.IsEmptyBucket(bucket)) {
skip_list_iter_.reset();
Reset(nullptr);
} else {
Node* first_linked_list_node =
memtable_rep_.GetLinkListFirstNode(bucket);
if (first_linked_list_node != nullptr) {
// The bucket is organized as a linked list
skip_list_iter_.reset();
Reset(first_linked_list_node);
HashLinkListRep::LinkListIterator::Seek(k, memtable_key);
} else {
IterKey encoded_key;
encoded_key.EncodeLengthPrefixedKey(k);
skip_list_iter_->Seek(encoded_key.GetUserKey().data());
SkipListBucketHeader* skip_list_header =
memtable_rep_.GetSkipListBucketHeader(bucket);
assert(skip_list_header != nullptr);
// The bucket is organized as a skip list
if (!skip_list_iter_) {
skip_list_iter_.reset(
new MemtableSkipList::Iterator(&skip_list_header->skip_list));
} else {
skip_list_iter_->SetList(&skip_list_header->skip_list);
}
if (memtable_key != nullptr) {
skip_list_iter_->Seek(memtable_key);
} else {
IterKey encoded_key;
encoded_key.EncodeLengthPrefixedKey(k);
skip_list_iter_->Seek(encoded_key.GetUserKey().data());
}
}
} else {
// The bucket is organized as a linked list
skip_list_iter_.reset();
Reset(memtable_rep_.GetLinkListFirstNode(bucket));
HashLinkListRep::LinkListIterator::Seek(k, memtable_key);
}
}
@ -528,36 +541,38 @@ KeyHandle HashLinkListRep::Allocate(const size_t len, char** buf) {
}
SkipListBucketHeader* HashLinkListRep::GetSkipListBucketHeader(
Pointer* first_next_pointer) const {
if (first_next_pointer == nullptr) {
return nullptr;
}
if (first_next_pointer->load(std::memory_order_relaxed) == nullptr) {
// Single entry bucket
return nullptr;
}
Pointer& bucket_pointer) const {
Pointer* first_next_pointer =
static_cast<Pointer*>(bucket_pointer.load(std::memory_order_acquire));
assert(first_next_pointer != nullptr);
assert(first_next_pointer->load(std::memory_order_relaxed) != nullptr);
// Counting header
BucketHeader* header = reinterpret_cast<BucketHeader*>(first_next_pointer);
if (header->IsSkipListBucket()) {
assert(header->GetNumEntries() > threshold_use_skiplist_);
auto* skip_list_bucket_header =
reinterpret_cast<SkipListBucketHeader*>(header);
assert(skip_list_bucket_header->Counting_header.next.load(
std::memory_order_relaxed) == header);
return skip_list_bucket_header;
}
assert(header->GetNumEntries() <= threshold_use_skiplist_);
return nullptr;
assert(header->IsSkipListBucket());
assert(header->GetNumEntries() > threshold_use_skiplist_);
auto* skip_list_bucket_header =
reinterpret_cast<SkipListBucketHeader*>(header);
assert(skip_list_bucket_header->Counting_header.next.load(
std::memory_order_relaxed) == header);
return skip_list_bucket_header;
}
Node* HashLinkListRep::GetLinkListFirstNode(Pointer* first_next_pointer) const {
if (first_next_pointer == nullptr) {
return nullptr;
}
Node* HashLinkListRep::GetLinkListFirstNode(Pointer& bucket_pointer) const {
Pointer* first_next_pointer =
static_cast<Pointer*>(bucket_pointer.load(std::memory_order_acquire));
assert(first_next_pointer != nullptr);
if (first_next_pointer->load(std::memory_order_relaxed) == nullptr) {
// Single entry bucket
return reinterpret_cast<Node*>(first_next_pointer);
}
// It is possible that after we fetch first_next_pointer it is modified
// and the next is not null anymore. In this case, the bucket should have been
// modified to a counting header, so we should reload the first_next_pointer
// to make sure we see the update.
first_next_pointer =
static_cast<Pointer*>(bucket_pointer.load(std::memory_order_acquire));
// Counting header
BucketHeader* header = reinterpret_cast<BucketHeader*>(first_next_pointer);
if (!header->IsSkipListBucket()) {
@ -695,17 +710,21 @@ bool HashLinkListRep::Contains(const char* key) const {
Slice internal_key = GetLengthPrefixedSlice(key);
auto transformed = GetPrefix(internal_key);
auto bucket = GetBucket(transformed);
if (bucket == nullptr) {
Pointer& bucket = GetBucket(transformed);
if (IsEmptyBucket(bucket)) {
return false;
}
Node* linked_list_node = GetLinkListFirstNode(bucket);
if (linked_list_node != nullptr) {
return LinkListContains(linked_list_node, internal_key);
}
SkipListBucketHeader* skip_list_header = GetSkipListBucketHeader(bucket);
if (skip_list_header != nullptr) {
return skip_list_header->skip_list.Contains(key);
} else {
return LinkListContains(GetLinkListFirstNode(bucket), internal_key);
}
return false;
}
size_t HashLinkListRep::ApproximateMemoryUsage() {
@ -716,21 +735,25 @@ size_t HashLinkListRep::ApproximateMemoryUsage() {
void HashLinkListRep::Get(const LookupKey& k, void* callback_args,
bool (*callback_func)(void* arg, const char* entry)) {
auto transformed = transform_->Transform(k.user_key());
auto bucket = GetBucket(transformed);
Pointer& bucket = GetBucket(transformed);
auto* skip_list_header = GetSkipListBucketHeader(bucket);
if (skip_list_header != nullptr) {
// Is a skip list
MemtableSkipList::Iterator iter(&skip_list_header->skip_list);
for (iter.Seek(k.memtable_key().data());
if (IsEmptyBucket(bucket)) {
return;
}
auto* link_list_head = GetLinkListFirstNode(bucket);
if (link_list_head != nullptr) {
LinkListIterator iter(this, link_list_head);
for (iter.Seek(k.internal_key(), nullptr);
iter.Valid() && callback_func(callback_args, iter.key());
iter.Next()) {
}
} else {
auto* link_list_head = GetLinkListFirstNode(bucket);
if (link_list_head != nullptr) {
LinkListIterator iter(this, link_list_head);
for (iter.Seek(k.internal_key(), nullptr);
auto* skip_list_header = GetSkipListBucketHeader(bucket);
if (skip_list_header != nullptr) {
// Is a skip list
MemtableSkipList::Iterator iter(&skip_list_header->skip_list);
for (iter.Seek(k.memtable_key().data());
iter.Valid() && callback_func(callback_args, iter.key());
iter.Next()) {
}
@ -746,25 +769,24 @@ MemTableRep::Iterator* HashLinkListRep::GetIterator(Arena* alloc_arena) {
for (size_t i = 0; i < bucket_size_; ++i) {
int count = 0;
auto* bucket = GetBucket(i);
if (bucket != nullptr) {
auto* skip_list_header = GetSkipListBucketHeader(bucket);
if (skip_list_header != nullptr) {
Pointer& bucket = GetBucket(i);
if (!IsEmptyBucket(bucket)) {
auto* link_list_head = GetLinkListFirstNode(bucket);
if (link_list_head != nullptr) {
LinkListIterator itr(this, link_list_head);
for (itr.SeekToHead(); itr.Valid(); itr.Next()) {
list->Insert(itr.key());
count++;
}
} else {
auto* skip_list_header = GetSkipListBucketHeader(bucket);
assert(skip_list_header != nullptr);
// Is a skip list
MemtableSkipList::Iterator itr(&skip_list_header->skip_list);
for (itr.SeekToFirst(); itr.Valid(); itr.Next()) {
list->Insert(itr.key());
count++;
}
} else {
auto* link_list_head = GetLinkListFirstNode(bucket);
if (link_list_head != nullptr) {
LinkListIterator itr(this, link_list_head);
for (itr.SeekToHead(); itr.Valid(); itr.Next()) {
list->Insert(itr.key());
count++;
}
}
}
}
if (if_log_bucket_dist_when_flash_) {

Write Preview
Loading…
Cancel
Save