elv_unregister: fix possible crash on module unload

An exiting task or process which didn't do I/O yet have no io context,
elv_unregister() should check it is not NULL.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Acked-by: Jens Axboe <axboe@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 2 insertions, 1 deletions

diff --git a/block/elevator.c b/block/elevator.c
index bc7baeec0d10..9b72dc7c8a5c 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -765,7 +765,8 @@ void elv_unregister(struct elevator_type *e)
                 read_lock(&tasklist_lock);
                 do_each_thread(g, p) {
                         task_lock(p);
-                        e->ops.trim(p->io_context);
+                        if (p->io_context)
+                                e->ops.trim(p->io_context);
                         task_unlock(p);
                 } while_each_thread(g, p);
                 read_unlock(&tasklist_lock);