diff --git a/src/pattern.c b/src/pattern.c
index a82900b..3dbc285 100644
--- a/src/pattern.c
+++ b/src/pattern.c
@@ -2100,18 +2100,27 @@
 }
 
 /* This function prune all entries of <ref>. This function
- * prune the associated pattern_expr.
+ * prunes the associated pattern_expr. It may return before the end of
+ * the list is reached, returning 0, to yield. The caller must call it
+ * again. Otherwise it returns 1 once done.
  */
-void pat_ref_prune(struct pat_ref *ref)
+int pat_ref_prune(struct pat_ref *ref)
 {
 	struct pat_ref_elt *elt, *safe;
 	struct pattern_expr *expr;
 	struct bref *bref, *back;
+	int loops = 0;
 
 	list_for_each_entry(expr, &ref->pat, list) {
 		HA_RWLOCK_WRLOCK(PATEXP_LOCK, &expr->lock);
 		expr->pat_head->prune(expr);
 		HA_RWLOCK_WRUNLOCK(PATEXP_LOCK, &expr->lock);
+		loops++;
+		/* yield often, some lists may be huge, especially those
+		 * having to be freed through free_pattern_tree()
+		 */
+		if (loops > 10)
+			return 0;
 	}
 
 	/* we trash pat_ref_elt in a second time to ensure that data is
@@ -2132,9 +2141,11 @@
 		free(elt->pattern);
 		free(elt->sample);
 		free(elt);
+		loops++;
+		if (loops > 100000)
+			return 0;
 	}
-
-
+	return 1;
 }
 
 /* This function lookup for existing reference <ref> in pattern_head <head>. */