diff --git a/db/db_impl.cc b/db/db_impl.cc
index 49b9595..c6da3a6 100644
--- a/db/db_impl.cc
+++ b/db/db_impl.cc
@@ -216,18 +216,26 @@ void DBImpl::MaybeIgnoreError(Status* s) const {
 }
 
 void DBImpl::DeleteObsoleteFiles() {
+  mutex_.AssertHeld();
+
   if (!bg_error_.ok()) {
     // After a background error, we don't know whether a new version may
     // or may not have been committed, so we cannot safely garbage collect.
     return;
   }
 
+  uint64_t log_number = versions_->LogNumber();
+  uint64_t prev_log_number = versions_->PrevLogNumber();
+  uint64_t manifest_file_number = versions_->ManifestFileNumber();
+
   // Make a set of all of the live files
   std::set<uint64_t> live = pending_outputs_;
   versions_->AddLiveFiles(&live);
 
   std::vector<std::string> filenames;
   env_->GetChildren(dbname_, &filenames); // Ignoring errors on purpose
+
+  mutex_.Unlock();
   uint64_t number;
   FileType type;
   for (size_t i = 0; i < filenames.size(); i++) {
@@ -235,13 +243,13 @@ void DBImpl::DeleteObsoleteFiles() {
       bool keep = true;
       switch (type) {
         case kLogFile:
-          keep = ((number >= versions_->LogNumber()) ||
-                  (number == versions_->PrevLogNumber()));
+          keep = ((number >= log_number) ||
+                  (number == prev_log_number));
           break;
         case kDescriptorFile:
           // Keep my manifest file, and any newer incarnations'
           // (in case there is a race that allows other incarnations)
-          keep = (number >= versions_->ManifestFileNumber());
+          keep = (number >= manifest_file_number);
           break;
         case kTableFile:
           keep = (live.find(number) != live.end());
@@ -269,6 +277,7 @@ void DBImpl::DeleteObsoleteFiles() {
       }
     }
   }
+  mutex_.Lock();
 }
 
 Status DBImpl::Recover(VersionEdit* edit) {