Re: [PATCH V6 05/10] signal: Perfom autoreap for PF_USER_WORKER

Eric W. Biederman
2021-12-17T18:43:47Z

Mike Christie <michael.christie@oracle.com> writes:

> Userspace doesn't know about PF_USER_WORKER threads, so it can't do wait
> to clean them up. For cases like where qemu will do dynamic/hot add/remove
> of vhost devices, then we need to auto reap the thread like was done for
> the kthread case, because qemu does not know what API the kernel/vhost
> layer is using.
>
> This has us do autoreaping for these threads similar to when the parent
> ignores SIGCHLD and for kthreads.

There is a lot wrong with this change.
1) you can just set "task->signal = SIGCHLD" to get this
behavior so it is unnecessary.

2) This is not the autoreaping you want. This autoreaping just kicks
in when the parents signal handler is SIG_IGN. Since I presume
you are not controlling the parent this is just plain nonsense.

The autoreap you want is the autoreap in exit_notify, and you don't
want to call do_notify_parent at all.

Eric

> Signed-off-by: Mike Christie <michael.christie@oracle.com>
> ---
> kernel/signal.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/signal.c b/kernel/signal.c
> index a629b11bf3e0..4ce2cc195269 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -2071,9 +2071,9 @@ bool do_notify_parent(struct task_struct *tsk, int sig)
>
> psig = tsk->parent->sighand;
> spin_lock_irqsave(&psig->siglock, flags);
> - if (!tsk->ptrace && sig == SIGCHLD &&
> + if (!tsk->ptrace && (tsk->flags & PF_USER_WORKER || (sig == SIGCHLD &&
> (psig->action[SIGCHLD-1].sa.sa_handler == SIG_IGN ||
> - (psig->action[SIGCHLD-1].sa.sa_flags & SA_NOCLDWAIT))) {
> + (psig->action[SIGCHLD-1].sa.sa_flags & SA_NOCLDWAIT))))) {
> /*
> * We are exiting and our parent doesn't care. POSIX.1
> * defines special semantics for setting SIGCHLD to SIG_IGN