[RocksDB] Fix MaybeDumpStats

Summary: MaybeDumpStats was causing lock problem

Test Plan: make check; db_stress

Reviewers: dhruba

Reviewed By: dhruba

Differential Revision: https://reviews.facebook.net/D10935

db/db_impl.cc

@@ -305,20 +305,21 @@ const Status DBImpl::CreateArchivalDirectory() {
 }
 
 void DBImpl::MaybeDumpStats() {
-  mutex_.AssertHeld();
+  if (options_.stats_dump_period_sec == 0) return;
 
-  if (options_.stats_dump_period_sec != 0) {
   const uint64_t now_micros = env_->NowMicros();
+
   if (last_stats_dump_time_microsec_ +
       options_.stats_dump_period_sec * 1000000
       <= now_micros) {
+    // Multiple threads could race in here simultaneously.
+    // However, the last one will update last_stats_dump_time_microsec_
+    // atomically. We could see more than one dump during one dump
+    // period in rare cases.
     last_stats_dump_time_microsec_ = now_micros;
-      mutex_.Unlock();
     std::string stats;
     GetProperty("leveldb.stats", &stats);
     Log(options_.info_log, "%s", stats.c_str());
-      mutex_.Lock();
-    }
   }
 }
 
@@ -1233,6 +1234,9 @@ void DBImpl::TEST_PurgeObsoleteteWAL() {
 void DBImpl::BackgroundCall() {
   bool madeProgress = false;
   DeletionState deletion_state;
+
+  MaybeDumpStats();
+
   MutexLock l(&mutex_);
   // Log(options_.info_log, "XXX BG Thread %llx process new work item", pthread_self());
   assert(bg_compaction_scheduled_);
@@ -1273,7 +1277,6 @@ void DBImpl::BackgroundCall() {
   }
   bg_cv_.SignalAll();
 
-  MaybeDumpStats();
 }
 
 Status DBImpl::BackgroundCompaction(bool* madeProgress,

db/db_impl.h

@@ -5,6 +5,7 @@
 #ifndef STORAGE_LEVELDB_DB_DB_IMPL_H_
 #define STORAGE_LEVELDB_DB_DB_IMPL_H_
 
+#include <atomic>
 #include <deque>
 #include <set>
 #include "db/dbformat.h"
@@ -277,7 +278,7 @@ class DBImpl : public DB {
   uint64_t purge_wal_files_last_run_;
 
   // last time stats were dumped to LOG
-  uint64_t last_stats_dump_time_microsec_;
+  std::atomic<uint64_t> last_stats_dump_time_microsec_;
 
   // These count the number of microseconds for which MakeRoomForWrite stalls.
   uint64_t stall_level0_slowdown_;