Core/PBX: Deadlock between dialplan execution and application unregistration.

Not easy to reproduce, but we have noticed deadlocks when unloading a module
while dialplan is handling a request.

The deadlock is between :
1) Dialplan execution: pbx_extension_helper() first taking conlock,
then pbx_findapp() [when called] asking for lock on apps list.
2) Application unregistration: ast_unregister_application() first taking lock
on apps list, then unreference_cached_app() [when called] asking for conlock.

As a protection, I suggest to modify ast_unregister_application(), so that it
anticipates the need of conlock, before taking the lock on apps list.
The side effect is a longer unavailability of conlock when unregistering an
application.

ASTERISK-27041

Change-Id: I0db0f1eb320da6a5758cce3a47d765be1face8e2

diff --git a/main/pbx_app.c b/main/pbx_app.c
index 1d90dac873d6b6e3d6e2e2f80278f8c7bb4cab61..1ce77122c5ff8baf5d48041271e18bbf55f99ac9 100644 (file)
--- a/main/pbx_app.c
+++ b/main/pbx_app.c
@@ -396,6 +396,11 @@ int ast_unregister_application(const char *app)
        struct ast_app *cur;
        int cmp;
 
+       /* Anticipate need for conlock in unreference_cached_app(), in order to avoid
+        * possible deadlock with pbx_extension_helper()/pbx_findapp()
+        */
+       ast_rdlock_contexts();
+
        AST_RWLIST_WRLOCK(&apps);
        AST_RWLIST_TRAVERSE_SAFE_BEGIN(&apps, cur, list) {
                cmp = strcasecmp(app, cur->name);
@@ -418,6 +423,8 @@ int ast_unregister_application(const char *app)
        AST_RWLIST_TRAVERSE_SAFE_END;
        AST_RWLIST_UNLOCK(&apps);
 
+       ast_unlock_contexts();
+
        return cur ? 0 : -1;
 }