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

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Message-id: <170181458198.7109.790647899711986334@noble.neil.brown.name>
Date:   Wed, 06 Dec 2023 09:16:21 +1100
From:   "NeilBrown" <neilb@...e.de>
To:     "Jens Axboe" <axboe@...nel.dk>
Cc:     "Christian Brauner" <brauner@...nel.org>,
        "Al Viro" <viro@...iv.linux.org.uk>,
        "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 Wed, 06 Dec 2023, Jens Axboe wrote:
> On 12/5/23 2:58 PM, Jens Axboe wrote:
> > On 12/5/23 2:28 PM, NeilBrown wrote:
> >> On Tue, 05 Dec 2023, 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
> > 
> >>>
> >>> 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.
> >>>
> >>
> >> Ok, maybe we need to reframe the problem then.
> >>
> >> Currently fput(), and hence filp_close(), take control away from kernel
> >> threads in that they cannot be sure that a "close" has actually
> >> completed.
> >>
> >> This is already a problem for nfsd.  When renaming a file, nfsd needs to
> >> ensure any cached "open" that it has on the file is closed (else when
> >> re-exporting an NFS filesystem it can result in a silly-rename).
> >>
> >> nfsd currently handles this case by calling flush_delayed_fput().  I
> >> suspect you are no more happy about exporting that than you are about
> >> exporting task_work_run(), but this solution isn't actually 100%
> >> reliable.  If some other thread calls flush_delayed_fput() between nfsd
> >> calling filp_close() and that same nfsd calling flush_delayed_fput(),
> >> then the second flush can return before the first flush (in the other
> >> thread) completes all the work it took on.
> >>
> >> What we really need - both for handling renames and for avoiding
> >> possible memory exhaustion - is for nfsd to be able to reliably wait for
> >> any fput() that it initiated to complete.
> >>
> >> How would you like the VFS to provide that service?
> > 
> > Since task_work happens in the context of your task already, why not
> > just have a way to get it stashed into a list when final fput is done?
> > This avoids all of this "let's expose task_work" and using the task list
> > for that, which seems kind of pointless as you're just going to run it
> > later on manually anyway.
> > 
> > In semi pseudo code:
> > 
> > bool fput_put_ref(struct file *file)
> > {
> > 	return atomic_dec_and_test(&file->f_count);
> > }
> > 
> > void fput(struct file *file)
> > {
> > 	if (fput_put_ref(file)) {
> > 		...
> > 	}
> > }
> > 
> > and then your nfsd_file_free() could do:
> > 
> > ret = filp_flush(file, id);
> > if (fput_put_ref(file))
> > 	llist_add(&file->f_llist, &l->to_free_llist);
> > 
> > or something like that, where l->to_free_llist is where ever you'd
> > otherwise punt the actual freeing to.
> 
> Should probably have the put_ref or whatever helper also init the
> task_work, and then reuse the list in the callback_head there. Then
> whoever flushes it has to call ->func() and avoid exposing ____fput() to
> random users. But you get the idea.

Interesting ideas - thanks.

So maybe the new API would be

 fput_queued(struct file *f, struct llist_head *q)
and
 flush_fput_queue(struct llist_head *q)

with the meaning being that fput_queued() is just like fput() except
that any file needing __fput() is added to the 'q'; and that
flush_fput_queue() calls __fput() on any files in 'q'.

So to close a file nfsd would:

  fget(f);
  flip_close(f);
  fput_queued(f, &my_queue);

though possibly we could have a
  filp_close_queued(f, q)
as well.

I'll try that out - but am happy to hear alternate suggestions for names :-)

Thanks,
NeilBrown