SERVER-33043 Only setFCV should take the fcvLock (ResourceMutex) in exclusive mode

2 files changed, 5 insertions, 1 deletions

diff --git a/src/mongo/db/commands/feature_compatibility_version.h b/src/mongo/db/commands/feature_compatibility_version.h
index 7634203531e..45a020c4762 100644
--- a/src/mongo/db/commands/feature_compatibility_version.h
+++ b/src/mongo/db/commands/feature_compatibility_version.h
@@ -56,8 +56,11 @@ public:
     static constexpr StringData kTargetVersionField = "targetVersion"_sd;
 
     /**
-     * Should be taken in exclusive mode by any operations that should not run while
+     * Should be taken in shared mode by any operations that should not run while
      * setFeatureCompatibilityVersion is running.
+     *
+     * setFCV takes this lock in exclusive mode so that it both does not run with the shared mode
+     * operations and does not run with itself.
      */
     static Lock::ResourceMutex fcvLock;
 
diff --git a/src/mongo/db/commands/set_feature_compatibility_version_command.cpp b/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
index 064a3a42cdd..cfce4a4967b 100644
--- a/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
+++ b/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
@@ -129,6 +129,7 @@ public:
         });
 
         // Only allow one instance of setFeatureCompatibilityVersion to run at a time.
+        invariant(!opCtx->lockState()->isLocked());
         Lock::ExclusiveLock lk(opCtx->lockState(), FeatureCompatibilityVersion::fcvLock);
 
         const auto requestedVersion = uassertStatusOK(