network/tc: Avoid concurrent set modification in tclass_drop()/qdisc_drop()

With the current algorithm, we can end up removing entries from the
qdisc/tclass sets while having multiple open iterators over the sets at
various positions which leads to assertion failures in the hashmap logic
as it's only safe to remove the "current" entry.

To avoid the problem, let's split up marking and dropping of tclasses
and qdiscs. First, we recursively iterate tclasses/qdiscs and mark all
that need to be removed. Next, we iterate once over tclasses and qdiscs
and remove all marked entries.

Fixes 632d321050

src/network/tc/qdisc.c

@@ -285,37 +285,57 @@ int link_find_qdisc(Link *link, uint32_t handle, const char *kind, QDisc **ret)
         return -ENOENT;
 }
 
-QDisc* qdisc_drop(QDisc *qdisc) {
+void qdisc_mark_recursive(QDisc *qdisc) {
         TClass *tclass;
-        Link *link;
 
         assert(qdisc);
+        assert(qdisc->link);
 
-        link = ASSERT_PTR(qdisc->link);
+        if (qdisc_is_marked(qdisc))
+                return;
 
-        qdisc_mark(qdisc); /* To avoid stack overflow. */
-
-        /* also drop all child classes assigned to the qdisc. */
-        SET_FOREACH(tclass, link->tclasses) {
-                if (tclass_is_marked(tclass))
-                        continue;
+        qdisc_mark(qdisc);
 
+        /* also mark all child classes assigned to the qdisc. */
+        SET_FOREACH(tclass, qdisc->link->tclasses) {
                 if (TC_H_MAJ(tclass->classid) != qdisc->handle)
                         continue;
 
-                tclass_drop(tclass);
+                tclass_mark_recursive(tclass);
         }
+}
 
-        qdisc_unmark(qdisc);
-        qdisc_enter_removed(qdisc);
+void link_qdisc_drop_marked(Link *link) {
+        QDisc *qdisc;
 
-        if (qdisc->state == 0) {
-                log_qdisc_debug(qdisc, link, "Forgetting");
-                qdisc = qdisc_free(qdisc);
-        } else
-                log_qdisc_debug(qdisc, link, "Removed");
+        assert(link);
 
-        return qdisc;
+        SET_FOREACH(qdisc, link->qdiscs) {
+                if (!qdisc_is_marked(qdisc))
+                        continue;
+
+                qdisc_unmark(qdisc);
+                qdisc_enter_removed(qdisc);
+
+                if (qdisc->state == 0) {
+                        log_qdisc_debug(qdisc, link, "Forgetting");
+                        qdisc_free(qdisc);
+                } else
+                        log_qdisc_debug(qdisc, link, "Removed");
+        }
+}
+
+QDisc* qdisc_drop(QDisc *qdisc) {
+        assert(qdisc);
+        assert(qdisc->link);
+
+        qdisc_mark_recursive(qdisc);
+
+        /* link_qdisc_drop_marked() may invalidate qdisc, so run link_tclass_drop_marked() first. */
+        link_tclass_drop_marked(qdisc->link);
+        link_qdisc_drop_marked(qdisc->link);
+
+        return NULL;
 }
 
 static int qdisc_handler(sd_netlink *rtnl, sd_netlink_message *m, Request *req, Link *link, QDisc *qdisc) {

src/network/tc/qdisc.h

@@ -77,7 +77,9 @@ DEFINE_NETWORK_CONFIG_STATE_FUNCTIONS(QDisc, qdisc);
 QDisc* qdisc_free(QDisc *qdisc);
 int qdisc_new_static(QDiscKind kind, Network *network, const char *filename, unsigned section_line, QDisc **ret);
 
+void qdisc_mark_recursive(QDisc *qdisc);
 QDisc* qdisc_drop(QDisc *qdisc);
+void link_qdisc_drop_marked(Link *link);
 
 int link_find_qdisc(Link *link, uint32_t handle, const char *kind, QDisc **qdisc);
 

src/network/tc/tclass.c

@@ -252,37 +252,56 @@ static void log_tclass_debug(TClass *tclass, Link *link, const char *str) {
                        strna(tclass_get_tca_kind(tclass)));
 }
 
-TClass* tclass_drop(TClass *tclass) {
+void tclass_mark_recursive(TClass *tclass) {
         QDisc *qdisc;
-        Link *link;
 
         assert(tclass);
+        assert(tclass->link);
 
-        link = ASSERT_PTR(tclass->link);
+        if (tclass_is_marked(tclass))
+                return;
 
-        tclass_mark(tclass); /* To avoid stack overflow. */
-
-        /* Also drop all child qdiscs assigned to the class. */
-        SET_FOREACH(qdisc, link->qdiscs) {
-                if (qdisc_is_marked(qdisc))
-                        continue;
+        tclass_mark(tclass);
 
+        /* Also mark all child qdiscs assigned to the class. */
+        SET_FOREACH(qdisc, tclass->link->qdiscs) {
                 if (qdisc->parent != tclass->classid)
                         continue;
 
-                qdisc_drop(qdisc);
+                qdisc_mark_recursive(qdisc);
         }
+}
 
-        tclass_unmark(tclass);
-        tclass_enter_removed(tclass);
+void link_tclass_drop_marked(Link *link) {
+        TClass *tclass;
 
-        if (tclass->state == 0) {
-                log_tclass_debug(tclass, link, "Forgetting");
-                tclass = tclass_free(tclass);
-        } else
-                log_tclass_debug(tclass, link, "Removed");
+        assert(link);
 
-        return tclass;
+        SET_FOREACH(tclass, link->tclasses) {
+                if (!tclass_is_marked(tclass))
+                        continue;
+
+                tclass_unmark(tclass);
+                tclass_enter_removed(tclass);
+
+                if (tclass->state == 0) {
+                        log_tclass_debug(tclass, link, "Forgetting");
+                        tclass_free(tclass);
+                } else
+                        log_tclass_debug(tclass, link, "Removed");
+        }
+}
+
+TClass* tclass_drop(TClass *tclass) {
+        assert(tclass);
+
+        tclass_mark_recursive(tclass);
+
+        /* link_tclass_drop_marked() may invalidate tclass, so run link_qdisc_drop_marked() first. */
+        link_qdisc_drop_marked(tclass->link);
+        link_tclass_drop_marked(tclass->link);
+
+        return NULL;
 }
 
 static int tclass_handler(sd_netlink *rtnl, sd_netlink_message *m, Request *req, Link *link, TClass *tclass) {

src/network/tc/tclass.h

@@ -58,7 +58,9 @@ DEFINE_NETWORK_CONFIG_STATE_FUNCTIONS(TClass, tclass);
 TClass* tclass_free(TClass *tclass);
 int tclass_new_static(TClassKind kind, Network *network, const char *filename, unsigned section_line, TClass **ret);
 
+void tclass_mark_recursive(TClass *tclass);
 TClass* tclass_drop(TClass *tclass);
+void link_tclass_drop_marked(Link *link);
 
 int link_find_tclass(Link *link, uint32_t classid, TClass **ret);