Index: milter-greylist-4.2.3/dkimcheck.c
===================================================================
--- milter-greylist-4.2.3.orig/dkimcheck.c
+++ milter-greylist-4.2.3/dkimcheck.c
@@ -63,6 +63,7 @@ __RCSID("$Id: dkimcheck.c,v 1.4 2008/10/
 #include "dkimcheck.h"
 
 static DKIM_LIB *dkim_ptr = NULL;
+static pthread_rwlock_t dkim_lock;
 static sfsistat dkimcheck_error(struct mlfi_priv *);
 
 static sfsistat
@@ -115,28 +116,36 @@ dkimcheck_error(priv)
 }
 
 void
-dkimcheck_init(void)
+dkimcheck_clear(void)
 {
+	/*
+	 * XXX This probably leaves stale handles for messages being processed
+	 */
+
+	WRLOCK(&dkim_lock);
+	if (dkim_ptr != NULL)
+		dkim_close(dkim_ptr);
+	dkim_ptr = NULL;
+
 	if ((dkim_ptr = dkim_init(NULL, NULL)) == NULL) {
 		mg_log(LOG_ERR, "dkim_init() failed");
 		exit(EX_OSERR);
 	}
-
-	return;
+	UNLOCK(&dkim_lock);
 }
 
 void
-dkimcheck_clear(void)
+dkimcheck_init(void)
 {
-	/*
-	 * XXX This probably leaves stale handles for messages being processed
-	 */
-	if (dkim_ptr != NULL)
-		dkim_close(dkim_ptr);
-	dkim_ptr = NULL;
+	int error;
 
-	dkimcheck_init();
-	return;
+	if ((error = pthread_rwlock_init(&dkim_lock, NULL)) != 0) {
+		mg_log(LOG_ERR, "pthread_rwlock_init failed: %s",
+		    strerror(error));
+		exit(EX_OSERR);
+	}
+
+	dkimcheck_clear();
 }
 
 sfsistat
@@ -159,8 +168,11 @@ dkimcheck_header(name, value, priv)
 		if (priv->priv_dkimstat != DKIM_STAT_OK)
 			return SMFIS_CONTINUE;
 
+		WRLOCK(&dkim_lock);
 		priv->priv_dkim = dkim_verify(dkim_ptr, priv->priv_queueid,
 					      NULL, &priv->priv_dkimstat);
+		UNLOCK(&dkim_lock);
+
 		if (priv->priv_dkim == NULL) {
 			mg_log(LOG_ERR, "dkim_verify() failed: %s",
 			       dkim_getresultstr(priv->priv_dkimstat));