linux-kernel - Re: [PATCH/RFC] core/nfsd: allow kernel threads to use task

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Message-id: <170112501017.7109.11367576354770728388@noble.neil.brown.name>
Date:   Tue, 28 Nov 2023 09:43:30 +1100
From:   "NeilBrown" <neilb@...e.de>
To:     "Al Viro" <viro@...iv.linux.org.uk>
Cc:     "Christian Brauner" <brauner@...nel.org>,
        "Jens Axboe" <axboe@...nel.dk>, "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/RFC] core/nfsd: allow kernel threads to use task_work.

On Tue, 28 Nov 2023, Al Viro wrote:
> 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()?

No locking. This is in the top level function of the kthread.
A ->release function that waits for an NFS filesystem to flush out data
through a filesystem exported by this nfsd might hit problems.
But that really requires us nfs-exporting and nfs filesystem which is
loop-back mounted.  While we do support nfs-reexport and nfs-loop-back
mounts, I don't think we make any pretence of supporting a combination.

Is that the sort of thing you were think of?

> 
> >  
> >  	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...
> 

I hadn't tried to understand what work_exited was for - but now I see
that its purpose is precisely to block further work from being queued -
exactly what I need.
Thanks - I make that change for a v2.

I've realised that I'll also need to change the flush_delayed_fput() in
fsd_file_close_inode_sync() to task_work_run().

Thanks,
NeilBrown