remove unnecessary code in super version getter (#11452)

Summary:
Do not bother comparing the version of the local super version handle with the global one.

An inequality comparison result indicates nothing but a spurious obsoleteness. It only happens when the writer has increased the `ColumnFamilyData::super_version_number_`(5fc57eec2b/db/column_family.cc (L1309)) but has not yet called `ResetThreadLocalSuperVersions()`(5fc57eec2b/db/column_family.cc (L1328)) at the time when a reader reads the local handle(`void* ptr = local_sv_->Swap(SuperVersion::kSVInUse);`). In other words, the existence of a local handle is a sufficent evidence of its fressness.

With this PR, we save one or even two atomic instructions when getting a handle of super version.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11452

Reviewed By: ajkr

Differential Revision: D46059317

Pulled By: cbi42

fbshipit-source-id: 68b4b1ca8a9929a4aa470105c37a09e0625b014d

db/column_family.cc

@@ -1245,30 +1245,11 @@ SuperVersion* ColumnFamilyData::GetThreadLocalSuperVersion(DBImpl* db) {
   // (if no Scrape happens).
   assert(ptr != SuperVersion::kSVInUse);
   SuperVersion* sv = static_cast<SuperVersion*>(ptr);
-  if (sv == SuperVersion::kSVObsolete ||
+  if (sv == SuperVersion::kSVObsolete) {
-      sv->version_number != super_version_number_.load()) {
     RecordTick(ioptions_.stats, NUMBER_SUPERVERSION_ACQUIRES);
-    SuperVersion* sv_to_delete = nullptr;
-
-    if (sv && sv->Unref()) {
-      RecordTick(ioptions_.stats, NUMBER_SUPERVERSION_CLEANUPS);
-      db->mutex()->Lock();
-      // NOTE: underlying resources held by superversion (sst files) might
-      // not be released until the next background job.
-      sv->Cleanup();
-      if (db->immutable_db_options().avoid_unnecessary_blocking_io) {
-        db->AddSuperVersionsToFreeQueue(sv);
-        db->SchedulePurge();
-      } else {
-        sv_to_delete = sv;
-      }
-    } else {
     db->mutex()->Lock();
-    }
     sv = super_version_->Ref();
     db->mutex()->Unlock();
-
-    delete sv_to_delete;
   }
   assert(sv != nullptr);
   return sv;