Fix a bug in hash linked list (#10401)

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

HISTORY.md

@@ -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

memtable/hash_linklist_rep.cc

@@ -209,10 +209,16 @@ class HashLinkListRep : public MemTableRep {
 
   bool LinkListContains(Node* head, const Slice& key) const;
 
-  SkipListBucketHeader* GetSkipListBucketHeader(Pointer* first_next_pointer)
+  bool IsEmptyBucket(Pointer& bucket_pointer) const {
-      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 {
+  Pointer& GetBucket(size_t i) const { return buckets_[i]; }
-    return static_cast<Pointer*>(buckets_[i].load(std::memory_order_acquire));
-  }
 
-  Pointer* GetBucket(const Slice& slice) const {
+  Pointer& GetBucket(const Slice& slice) const {
     return GetBucket(GetHash(slice));
   }
 
@@ -414,11 +418,24 @@ 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);
+      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 {
           SkipListBucketHeader* skip_list_header =
               memtable_rep_.GetSkipListBucketHeader(bucket);
-      if (skip_list_header != nullptr) {
+          assert(skip_list_header != nullptr);
           // The bucket is organized as a skip list
           if (!skip_list_iter_) {
             skip_list_iter_.reset(
@@ -433,11 +450,7 @@ class HashLinkListRep : public MemTableRep {
             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);
       }
     }
 
@@ -528,17 +541,15 @@ KeyHandle HashLinkListRep::Allocate(const size_t len, char** buf) {
 }
 
 SkipListBucketHeader* HashLinkListRep::GetSkipListBucketHeader(
-    Pointer* first_next_pointer) const {
+    Pointer& bucket_pointer) const {
-  if (first_next_pointer == nullptr) {
+  Pointer* first_next_pointer =
-    return nullptr;
+      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) {
+  assert(first_next_pointer->load(std::memory_order_relaxed) != nullptr);
-    // Single entry bucket
+
-    return nullptr;
-  }
   // Counting header
   BucketHeader* header = reinterpret_cast<BucketHeader*>(first_next_pointer);
-  if (header->IsSkipListBucket()) {
+  assert(header->IsSkipListBucket());
   assert(header->GetNumEntries() > threshold_use_skiplist_);
   auto* skip_list_bucket_header =
       reinterpret_cast<SkipListBucketHeader*>(header);
@@ -546,18 +557,22 @@ SkipListBucketHeader* HashLinkListRep::GetSkipListBucketHeader(
              std::memory_order_relaxed) == header);
   return skip_list_bucket_header;
 }
-  assert(header->GetNumEntries() <= threshold_use_skiplist_);
-  return nullptr;
-}
 
-Node* HashLinkListRep::GetLinkListFirstNode(Pointer* first_next_pointer) const {
+Node* HashLinkListRep::GetLinkListFirstNode(Pointer& bucket_pointer) const {
-  if (first_next_pointer == nullptr) {
+  Pointer* first_next_pointer =
-    return nullptr;
+      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);
+  Pointer& bucket = GetBucket(transformed);
-  if (bucket == nullptr) {
+  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() {
@@ -716,17 +735,12 @@ 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 (IsEmptyBucket(bucket)) {
-  if (skip_list_header != nullptr) {
+    return;
-    // 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()) {
   }
-  } else {
+
   auto* link_list_head = GetLinkListFirstNode(bucket);
   if (link_list_head != nullptr) {
     LinkListIterator iter(this, link_list_head);
@@ -734,6 +748,15 @@ void HashLinkListRep::Get(const LookupKey& k, void* callback_args,
          iter.Valid() && callback_func(callback_args, iter.key());
          iter.Next()) {
     }
+  } else {
+    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()) {
+      }
     }
   }
 }
@@ -746,17 +769,8 @@ MemTableRep::Iterator* HashLinkListRep::GetIterator(Arena* alloc_arena) {
 
   for (size_t i = 0; i < bucket_size_; ++i) {
     int count = 0;
-    auto* bucket = GetBucket(i);
+    Pointer& bucket = GetBucket(i);
-    if (bucket != nullptr) {
+    if (!IsEmptyBucket(bucket)) {
-      auto* skip_list_header = GetSkipListBucketHeader(bucket);
-      if (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);
@@ -764,6 +778,14 @@ MemTableRep::Iterator* HashLinkListRep::GetIterator(Arena* alloc_arena) {
           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++;
           }
       }
     }