On Mon, Aug 14, 2023 at 04:35:56PM +0200, Miklos Szeredi wrote: > On Mon, 14 Aug 2023 at 16:00, Tycho Andersen <tycho@tycho.pizza> wrote: > > > It seems like we really do need to wait here. I guess that means we > > need some kind of exit-proof wait? > > Could you please recap the original problem? Sure, the symptom is a deadlock, something like: # cat /proc/1528591/stack [<0>] do_wait+0x156/0x2f0 [<0>] kernel_wait4+0x8d/0x140 [<0>] zap_pid_ns_processes+0x104/0x180 [<0>] do_exit+0xa41/0xb80 [<0>] do_group_exit+0x3a/0xa0 [<0>] __x64_sys_exit_group+0x14/0x20 [<0>] do_syscall_64+0x37/0xb0 [<0>] entry_SYSCALL_64_after_hwframe+0x44/0xae which is stuck waiting for: # cat /proc/1544574/stack [<0>] request_wait_answer+0x12f/0x210 [<0>] fuse_simple_request+0x109/0x2c0 [<0>] fuse_flush+0x16f/0x1b0 [<0>] filp_close+0x27/0x70 [<0>] put_files_struct+0x6b/0xc0 [<0>] do_exit+0x360/0xb80 [<0>] do_group_exit+0x3a/0xa0 [<0>] get_signal+0x140/0x870 [<0>] arch_do_signal_or_restart+0xae/0x7c0 [<0>] exit_to_user_mode_prepare+0x10f/0x1c0 [<0>] syscall_exit_to_user_mode+0x26/0x40 [<0>] do_syscall_64+0x46/0xb0 [<0>] entry_SYSCALL_64_after_hwframe+0x44/0xae I have a reproducer here: https://github.com/tych0/kernel-utils/blob/master/fuse2/Makefile#L7 The problem is that the second thread has called do_exit() -> exit_signals(), but then tries to request_wait_answer() which uses the core wait primitives that no longer get woken up from signals due to the code in exit_signals(). So when we try to exit the pid ns, the whole cleanup hangs. It seems we really do need to wait in do_exit(), otherwise we get the behavior described in this regression... Tycho
