--- mod_security-2.9.1/apache2/modsecurity.h
+++ mod_security-2.9.1/apache2/modsecurity.h
@@ -657,6 +657,7 @@
     apr_pool_t              *mp;
     apr_global_mutex_t      *auditlog_lock;
     apr_global_mutex_t      *geo_lock;
+    apr_thread_mutex_t      *dbm_lock;
     msre_engine             *msre;
     unsigned int             processing_mode;
 };
--- mod_security-2.9.1/apache2/modsecurity.c
+++ mod_security-2.9.1/apache2/modsecurity.c
@@ -178,22 +178,30 @@
  * Performs per-child (new process) initialisation.
  */
 void modsecurity_child_init(msc_engine *msce) {
+    apr_status_t rc;
     /* Need to call this once per process before any other XML calls. */
     xmlInitParser();
 
     if (msce->auditlog_lock != NULL) {
-        apr_status_t rc = apr_global_mutex_child_init(&msce->auditlog_lock, NULL, msce->mp);
+        rc = apr_global_mutex_child_init(&msce->auditlog_lock, NULL, msce->mp);
         if (rc != APR_SUCCESS) {
             // ap_log_error(APLOG_MARK, APLOG_ERR, rs, s, "Failed to child-init auditlog mutex");
         }
     }
 
     if (msce->geo_lock != NULL) {
-        apr_status_t rc = apr_global_mutex_child_init(&msce->geo_lock, NULL, msce->mp);
+        rc = apr_global_mutex_child_init(&msce->geo_lock, NULL, msce->mp);
         if (rc != APR_SUCCESS) {
             // ap_log_error(APLOG_MARK, APLOG_ERR, rs, s, "Failed to child-init geo mutex");
         }
     }
+    
+    rc = apr_thread_mutex_create(&msce->dbm_lock,
+                                 APR_THREAD_MUTEX_DEFAULT, msce->mp);
+    if (rc != APR_SUCCESS) {
+        // ap_log_error(APLOG_MARK, APLOG_ERR, rs, s, "Failed to create dbm mutex");
+    }
+
 
 }
 
@@ -213,6 +221,7 @@
     apr_time_t time_before, time_after;
     int i;
 
+    apr_thread_mutex_lock(msr->modsecurity->dbm_lock);
     time_before = apr_time_now();
 
     /* Collections, store & remove stale. */
@@ -251,6 +260,8 @@
                 " microseconds.", msr->time_gc);
         }
     }
+    apr_thread_mutex_unlock(msr->modsecurity->dbm_lock);
+   
 }
 
 /**
--- mod_security-2.9.1/apache2/re_actions.c
+++ mod_security-2.9.1/apache2/re_actions.c
@@ -1951,6 +1951,7 @@
         /* ENH Warn about this. */
         return 0;
     }
+    apr_thread_mutex_lock(msr->modsecurity->dbm_lock);
 
     /* Init collection from storage. */
     table = collection_retrieve(msr, real_col_name, col_key, col_key_len);
@@ -1964,8 +1965,11 @@
         }
 
         table = apr_table_make(msr->mp, 24);
-        if (table == NULL) return -1;
-
+        if (table == NULL) {
+            
+            apr_thread_mutex_unlock(msr->modsecurity->dbm_lock);
+            return -1;
+        }
         /* IMP1 Is the timeout hard-coded to 3600? */
 
        if(msr->txcfg->debuglog_level >= 4) {
@@ -2063,6 +2067,7 @@
                 log_escape(msr->mp, real_col_name));
         }
     }
+    apr_thread_mutex_unlock(msr->modsecurity->dbm_lock);
 
     return 1;
 }