linux-kernel - Re: [PATCH 1/2] Allow a kthread to declare that it calls task_work

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Date:   Tue, 05 Dec 2023 09:06:05 -0500
From:   Jeff Layton <jlayton@...nel.org>
To:     Christian Brauner <brauner@...nel.org>,
        Jens Axboe <axboe@...nel.dk>
Cc:     NeilBrown <neilb@...e.de>, Al Viro <viro@...iv.linux.org.uk>,
        Oleg Nesterov <oleg@...hat.com>,
        Chuck Lever <chuck.lever@...cle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nfs@...r.kernel.org
Subject: Re: [PATCH 1/2] Allow a kthread to declare that it calls
 task_work_run()

On Tue, 2023-12-05 at 12:14 +0100, Christian Brauner wrote:
> On Mon, Dec 04, 2023 at 03:09:44PM -0700, Jens Axboe wrote:
> > On 12/4/23 2:02 PM, NeilBrown wrote:
> > > It isn't clear to me what _GPL is appropriate, but maybe the rules
> > > changed since last I looked..... are there rules?
> > > 
> > > My reasoning was that the call is effectively part of the user-space
> > > ABI.  A user-space process can call this trivially by invoking any
> > > system call.  The user-space ABI is explicitly a boundary which the GPL
> > > does not cross.  So it doesn't seem appropriate to prevent non-GPL
> > > kernel code from doing something that non-GPL user-space code can
> > > trivially do.
> > 
> > By that reasoning, basically everything in the kernel should be non-GPL
> > marked. And while task_work can get used by the application, it happens
> > only indirectly or implicitly. So I don't think this reasoning is sound
> > at all, it's not an exported ABI or API by itself.
> > 
> > For me, the more core of an export it is, the stronger the reason it
> > should be GPL. FWIW, I don't think exporting task_work functionality is
> > a good idea in the first place, but if there's a strong reason to do so,
> 
> Yeah, I'm not too fond of that part as well. I don't think we want to
> give modules the ability to mess with task work. This is just asking for
> trouble.

The fact that nfsd has to queue all of the delayed fput activity to a
workqueue has always been a horrible hack though. We export all kinds of
functionality to modules that you can screw up.

I think that nfsd's use-case is legitimate. ksmbd may also want to
follow suit.
-- 
Jeff Layton <jlayton@...nel.org>