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

From: Al Viro <viro@zeniv.linux.org.uk>
To: NeilBrown <neilb@suse.de>
Cc: Christian Brauner <brauner@kernel.org>,
	Jens Axboe <axboe@kernel.dk>, Oleg Nesterov <oleg@redhat.com>,
	Chuck Lever <chuck.lever@oracle.com>,
	Jeff Layton <jlayton@kernel.org>, Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-nfs@vger.kernel.org
Subject: Re: [PATCH/RFC] core/nfsd: allow kernel threads to use task_work.
Date: Mon, 27 Nov 2023 22:30:54 +0000	[thread overview]
Message-ID: <20231127223054.GL38156@ZenIV> (raw)
In-Reply-To: <170112272125.7109.6245462722883333440@noble.neil.brown.name>

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...