fix deadlock with enable_pipelined_write=true and max_successive_merges > 0

Summary:
fix this https://github.com/facebook/rocksdb/issues/3916
Closes https://github.com/facebook/rocksdb/pull/3923

Differential Revision: D8215192

Pulled By: yiwu-arbug

fbshipit-source-id: a4c2f839a91d92dc70906d2b7c6de0fe014a2422

HISTORY.md

@@ -7,6 +7,9 @@
 ### New Features
 * Changes the format of index blocks by storing the key in their raw form rather than converting them to InternalKey. This saves 8 bytes per index key. The feature is backward compatbile but not forward compatible. It is disabled by default unless format_version 3 or above is used.
 
+### Bug Fixes
+* fix deadlock with enable_pipelined_write=true and max_successive_merges > 0
+
 ## 5.14.0 (5/16/2018)
 ### Public API Change
 * Add a BlockBasedTableOption to align uncompressed data blocks on the smaller of block size or page size boundary, to reduce flash reads by avoiding reads spanning 4K pages.

db/db_impl_write.cc

@@ -1281,7 +1281,11 @@ Status DBImpl::SwitchMemtable(ColumnFamilyData* cfd, WriteContext* context,
   // In case of pipelined write is enabled, wait for all pending memtable
   // writers.
   if (immutable_db_options_.enable_pipelined_write) {
+    // Memtable writers may call DB::Get in case max_successive_merges > 0,
+    // which may lock mutex. Unlocking mutex here to avoid deadlock.
+    mutex_.Unlock();
     write_thread_.WaitForMemTableWriters();
+    mutex_.Lock();
   }
 
   // Attempt to switch to a new memtable and trigger flush of old.