Re: [PATCH/RFC] core/nfsd: allow kernel threads to use task_work.

[Al Viro <viro@zeniv.linux.org.uk>]

On Tue, Nov 28, 2023 at 09:05:21AM +1100, NeilBrown wrote:

> A simple way to fix this is to treat nfsd threads like normal processes
> for task_work.  Thus the pending files are queued for the thread, and
> the same thread finishes the work.
> 
> Currently KTHREADs are assumed never to call task_work_run().  With this
> patch that it still the default but it is implemented by storing the
> magic value TASK_WORKS_DISABLED in ->task_works.  If a kthread, such as
> nfsd, will call task_work_run() periodically, it sets ->task_works
> to NULL to indicate this.

>  		svc_recv(rqstp);
>  		validate_process_creds();
> +		if (task_work_pending(current))
> +			task_work_run();

What locking environment and call chain do you have here?  And what happens if
you get something stuck in ->release()?

>  
>  	p->pdeath_signal = 0;
> -	p->task_works = NULL;
> +	p->task_works = args->kthread ? TASK_WORKS_DISABLED : NULL;

Umm... why not have them set (by helper in kernel/task_work.c) to
&work_exited?  Then the task_work_run parts wouldn't be needed at all...