Re: [PATCH 09/18] io-wq: fork worker threads from original task

[Stefan Metzmacher <metze@samba.org>]

Hi Jens,

>> Can you please explain why CLONE_SIGHAND is used here?
> 
> We can't have CLONE_THREAD without CLONE_SIGHAND... The io-wq workers
> don't really care about signals, we don't use them internally.

I'm 100% sure, but I heard rumors that in some situations signals get
randomly delivered to any thread of a userspace process.

My fear was that the related logic may select a kernel thread if they
share the same signal handlers.

>> Will the userspace signal handlers executed from the kernel thread?
> 
> No

Good.

Are these threads immutable against signals from userspace?

>> Will SIGCHLD be posted to the userspace signal handlers in a userspace
>> process? Will wait() from userspace see the exit of a thread?
> 
> Currently actually it does, but I think that's just an oversight. As far
> as I can tell, we want to add something like the below. Untested... I'll
> give this a spin in a bit.
> 
> diff --git a/kernel/signal.c b/kernel/signal.c
> index ba4d1ef39a9e..e5db1d8f18e5 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -1912,6 +1912,10 @@ bool do_notify_parent(struct task_struct *tsk, int sig)
>  	bool autoreap = false;
>  	u64 utime, stime;
>  
> +	/* Don't notify a parent task if an io_uring worker exits */
> +	if (tsk->flags & PF_IO_WORKER)
> +		return true;
> +
>  	BUG_ON(sig == -1);
>  
>   	/* do_notify_parent_cldstop should have been called instead.  */
> 

Ok, thanks!

metze