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

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Message-id: <170172883971.7109.15813436459590259773@noble.neil.brown.name>
Date:   Tue, 05 Dec 2023 09:27:19 +1100
From:   "NeilBrown" <neilb@...e.de>
To:     "Jens Axboe" <axboe@...nel.dk>
Cc:     "Al Viro" <viro@...iv.linux.org.uk>,
        "Christian Brauner" <brauner@...nel.org>,
        "Oleg Nesterov" <oleg@...hat.com>,
        "Chuck Lever" <chuck.lever@...cle.com>,
        "Jeff Layton" <jlayton@...nel.org>,
        "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, 05 Dec 2023, 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,
> it should most certainly not be accessible to non-GPL modules. Basically
> NO new export should be non-GPL.

An alternate to exporting task_work_run() might be to call it from
try_to_freeze().  I think that too should only be called from a context
where no locks are held etc.  Obviously try_to_freeze would only call
task_work_run() if PF_RUNS_TASK_WORK were set.
I'm not sure this is a *good* idea, but it is an idea that would avoid
the export.

For now I change the export to _GPL.

Thanks,
NeilBrown