Fix a race in LRUCacheShard::Promote (#8717)

Summary:
In ```LRUCacheShard::Promote```, a reference is released outside the LRU mutex. Fix the race condition.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8717

Reviewed By: zhichao-cao

Differential Revision: D30649206

Pulled By: anand1976

fbshipit-source-id: 09c0af05b2294a7fe2c02876a61b0bad6e3ada61

HISTORY.md

@@ -5,6 +5,7 @@
 * Fixed a bug of secondary instance's last_sequence going backward, and reads on the secondary fail to see recent updates from the primary.
 * Fixed a bug that could lead to duplicate DB ID or DB session ID in POSIX environments without /proc/sys/kernel/random/uuid.
 * Fix a race in DumpStats() with column family destruction due to not taking a Ref on each entry while iterating the ColumnFamilySet.
+* Fix a race in item ref counting in LRUCache when promoting an item from the SecondaryCache.
 
 ### New Features
 * RemoteCompaction's interface now includes `db_name`, `db_id`, `session_id`, which could help the user uniquely identify compaction job between db instances and sessions.

cache/lru_cache.cc

@@ -360,7 +360,10 @@ Status LRUCacheShard::InsertItem(LRUHandle* e, Cache::Handle** handle,
       if (handle == nullptr) {
         LRU_Insert(e);
       } else {
+        // If caller already holds a ref, no need to take one here
+        if (!e->HasRefs()) {
           e->Ref();
+        }
         *handle = reinterpret_cast<Cache::Handle*>(e);
       }
     }
@@ -398,11 +401,7 @@ void LRUCacheShard::Promote(LRUHandle* e) {
   if (e->value) {
     Cache::Handle* handle = reinterpret_cast<Cache::Handle*>(e);
     Status s = InsertItem(e, &handle, /*free_handle_on_fail=*/false);
-    if (s.ok()) {
-      // InsertItem would have taken a reference on the item, so decrement it
-      // here as we expect the caller to already hold a reference
-      e->Unref();
-    } else {
+    if (!s.ok()) {
       // Item is in memory, but not accounted against the cache capacity.
       // When the handle is released, the item should get deleted
       assert(!e->InCache());