Fix a race in ColumnFamilyData::UnrefAndTryDelete (#8605)

Summary:
The `ColumnFamilyData::UnrefAndTryDelete` code currently on the trunk
unlocks the DB mutex before destroying the `ThreadLocalPtr` holding
the per-thread `SuperVersion` pointers when the only remaining reference
is the back reference from `super_version_`. The idea behind this was to
break the circular dependency between `ColumnFamilyData` and `SuperVersion`:
when the penultimate reference goes away, `ColumnFamilyData` can clean up
the `SuperVersion`, which can in turn clean up `ColumnFamilyData`. (Assuming there
is a `SuperVersion` and it is not referenced by anything else.) However,
unlocking the mutex throws a wrench in this plan by making it possible for another thread
to jump in and take another reference to the `ColumnFamilyData`, keeping the
object alive in a zombie `ThreadLocalPtr`-less state. This can cause issues like
#8440 ,
#8382 ,
and might also explain the `was_last_ref` assertion failures from the `ColumnFamilySet`
destructor we sometimes observe during close in our stress tests.

Digging through the archives, this unlocking goes way back to 2014 (or earlier). The original
rationale was that `SuperVersionUnrefHandle` used to lock the mutex so it can call
`SuperVersion::Cleanup`; however, this logic turned out to be deadlock-prone.
#3510 fixed the deadlock but left the
unlocking in place. #6147 then introduced
the circular dependency and associated cleanup logic described above (in order
to enable iterators to keep the `ColumnFamilyData` for dropped column families alive),
and moved the unlocking-relocking snippet to its present location in `UnrefAndTryDelete`.
Finally, #7749 fixed a memory leak but
apparently exacerbated the race by (otherwise correctly) switching to `UnrefAndTryDelete`
in `SuperVersion::Cleanup`.

The patch simply eliminates the unlocking and relocking, which has been unnecessary
ever since #3510 made `SuperVersionUnrefHandle` lock-free.
This closes the window during which another thread could increase the reference count,
and hopefully fixes the issues above.

Pull Request resolved: #8605

Test Plan: Ran `make check` and stress tests locally.

Reviewed By: pdillinger

Differential Revision: D30051035

Pulled By: ltamasi

fbshipit-source-id: 8fe559e4b4ad69fc142579f8bc393ef525918528

db/column_family.cc

@@ -457,7 +457,7 @@ void SuperVersion::Cleanup() {
     to_delete.push_back(m);
   }
   current->Unref();
-  cfd->UnrefAndTryDelete(this);
+  cfd->UnrefAndTryDelete();
 }
 
 void SuperVersion::Init(ColumnFamilyData* new_cfd, MemTable* new_mem,
@@ -475,10 +475,10 @@ void SuperVersion::Init(ColumnFamilyData* new_cfd, MemTable* new_mem,
 
 namespace {
 void SuperVersionUnrefHandle(void* ptr) {
-  // UnrefHandle is called when a thread exists or a ThreadLocalPtr gets
-  // destroyed. When former happens, the thread shouldn't see kSVInUse.
-  // When latter happens, we are in ~ColumnFamilyData(), no get should happen as
-  // well.
+  // UnrefHandle is called when a thread exits or a ThreadLocalPtr gets
+  // destroyed. When the former happens, the thread shouldn't see kSVInUse.
+  // When the latter happens, only super_version_ holds a reference
+  // to ColumnFamilyData, so no further queries are possible.
   SuperVersion* sv = static_cast<SuperVersion*>(ptr);
   bool was_last_ref __attribute__((__unused__));
   was_last_ref = sv->Unref();
@@ -668,7 +668,7 @@ ColumnFamilyData::~ColumnFamilyData() {
   }
 }
 
-bool ColumnFamilyData::UnrefAndTryDelete(SuperVersion* sv_under_cleanup) {
+bool ColumnFamilyData::UnrefAndTryDelete() {
   int old_refs = refs_.fetch_sub(1);
   assert(old_refs > 0);
 
@@ -678,22 +678,17 @@ bool ColumnFamilyData::UnrefAndTryDelete(SuperVersion* sv_under_cleanup) {
     return true;
   }
 
-  // If called under SuperVersion::Cleanup, we should not re-enter Cleanup on
-  // the same SuperVersion. (But while installing a new SuperVersion, this
-  // cfd could be referenced only by two SuperVersions.)
-  if (old_refs == 2 && super_version_ != nullptr &&
-      super_version_ != sv_under_cleanup) {
+  if (old_refs == 2 && super_version_ != nullptr) {
     // Only the super_version_ holds me
     SuperVersion* sv = super_version_;
     super_version_ = nullptr;
-    // Release SuperVersion reference kept in ThreadLocalPtr.
-    // This must be done outside of mutex_ since unref handler can lock mutex.
-    sv->db_mutex->Unlock();
+
+    // Release SuperVersion references kept in ThreadLocalPtr.
     local_sv_.reset();
-    sv->db_mutex->Lock();
 
     if (sv->Unref()) {
-      // May delete this ColumnFamilyData after calling Cleanup()
+      // Note: sv will delete this ColumnFamilyData during Cleanup()
+      assert(sv->cfd == this);
       sv->Cleanup();
       delete sv;
       return true;
@@ -1261,14 +1256,13 @@ bool ColumnFamilyData::ReturnThreadLocalSuperVersion(SuperVersion* sv) {
 void ColumnFamilyData::InstallSuperVersion(
     SuperVersionContext* sv_context, InstrumentedMutex* db_mutex) {
   db_mutex->AssertHeld();
-  return InstallSuperVersion(sv_context, db_mutex, mutable_cf_options_);
+  return InstallSuperVersion(sv_context, mutable_cf_options_);
 }
 
 void ColumnFamilyData::InstallSuperVersion(
-    SuperVersionContext* sv_context, InstrumentedMutex* db_mutex,
+    SuperVersionContext* sv_context,
     const MutableCFOptions& mutable_cf_options) {
   SuperVersion* new_superversion = sv_context->new_superversion.release();
-  new_superversion->db_mutex = db_mutex;
   new_superversion->mutable_cf_options = mutable_cf_options;
   new_superversion->Init(this, mem_, imm_.current(), current_);
   SuperVersion* old_superversion = super_version_;

db/column_family.h

@@ -208,8 +208,6 @@ struct SuperVersion {
   uint64_t version_number;
   WriteStallCondition write_stall_condition;
 
-  InstrumentedMutex* db_mutex;
-
   // should be called outside the mutex
   SuperVersion() = default;
   ~SuperVersion();
@@ -281,8 +279,7 @@ class ColumnFamilyData {
   // UnrefAndTryDelete() decreases the reference count and do free if needed,
   // return true if this is freed else false, UnrefAndTryDelete() can only
   // be called while holding a DB mutex, or during single-threaded recovery.
-  // sv_under_cleanup is only provided when called from SuperVersion::Cleanup.
-  bool UnrefAndTryDelete(SuperVersion* sv_under_cleanup = nullptr);
+  bool UnrefAndTryDelete();
 
   // SetDropped() can only be called under following conditions:
   // 1) Holding a DB mutex,
@@ -454,7 +451,6 @@ class ColumnFamilyData {
   // the clients to allocate SuperVersion outside of mutex.
   // IMPORTANT: Only call this from DBImpl::InstallSuperVersion()
   void InstallSuperVersion(SuperVersionContext* sv_context,
-                           InstrumentedMutex* db_mutex,
                            const MutableCFOptions& mutable_cf_options);
   void InstallSuperVersion(SuperVersionContext* sv_context,
                            InstrumentedMutex* db_mutex);

db/db_impl/db_impl_compaction_flush.cc

@@ -3467,7 +3467,7 @@ void DBImpl::InstallSuperVersionAndScheduleWork(
   if (UNLIKELY(sv_context->new_superversion == nullptr)) {
     sv_context->NewSuperVersion();
   }
-  cfd->InstallSuperVersion(sv_context, &mutex_, mutable_cf_options);
+  cfd->InstallSuperVersion(sv_context, mutable_cf_options);
 
   // There may be a small data race here. The snapshot tricking bottommost
   // compaction may already be released here. But assuming there will always be