diff --git a/src/sync.h b/src/sync.h
--- a/src/sync.h
+++ b/src/sync.h
@@ -67,6 +67,8 @@
 std::string LocksHeld();
 void AssertLockHeldInternal(const char *pszName, const char *pszFile, int nLine,
                             void *cs);
+void AssertLockNotHeldInternal(const char *pszName, const char *pszFile,
+                               int nLine, void *cs);
 void DeleteLock(void *cs);
 #else
 static inline void EnterCritical(const char *pszName, const char *pszFile,
@@ -75,9 +77,14 @@
 static inline void AssertLockHeldInternal(const char *pszName,
                                           const char *pszFile, int nLine,
                                           void *cs) {}
+static inline void AssertLockNotHeldInternal(const char *pszName,
+                                             const char *pszFile, int nLine,
+                                             void *cs) {}
 static inline void DeleteLock(void *cs) {}
 #endif
 #define AssertLockHeld(cs) AssertLockHeldInternal(#cs, __FILE__, __LINE__, &cs)
+#define AssertLockNotHeld(cs)                                                  \
+    AssertLockNotHeldInternal(#cs, __FILE__, __LINE__, &cs)
 
 /**
  * Wrapped mutex: supports recursive locking, but no waiting
diff --git a/src/sync.cpp b/src/sync.cpp
--- a/src/sync.cpp
+++ b/src/sync.cpp
@@ -158,6 +158,18 @@
     abort();
 }
 
+void AssertLockNotHeldInternal(const char *pszName, const char *pszFile,
+                               int nLine, void *cs) {
+    for (const std::pair<void *, CLockLocation> &i : *lockstack) {
+        if (i.first == cs) {
+            fprintf(stderr,
+                    "Assertion failed: lock %s held in %s:%i; locks held:\n%s",
+                    pszName, pszFile, nLine, LocksHeld().c_str());
+            abort();
+        }
+    }
+}
+
 void DeleteLock(void *cs) {
     if (!lockdata.available) {
         // We're already shutting down.