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

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Message-id: <170172377302.7109.11739406555273171485@noble.neil.brown.name>
Date:   Tue, 05 Dec 2023 08:02:53 +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 Mon, 04 Dec 2023, Jens Axboe wrote:
> On 12/3/23 6:36 PM, NeilBrown wrote:
> > diff --git a/fs/namespace.c b/fs/namespace.c
> > index e157efc54023..46d640b70ca9 100644
> > --- a/fs/namespace.c
> > +++ b/fs/namespace.c
> > @@ -1328,7 +1328,7 @@ static void mntput_no_expire(struct mount *mnt)
> >  
> >  	if (likely(!(mnt->mnt.mnt_flags & MNT_INTERNAL))) {
> >  		struct task_struct *task = current;
> > -		if (likely(!(task->flags & PF_KTHREAD))) {
> > +		if (likely((task->flags & PF_RUNS_TASK_WORK))) {
> 
> Extraneous parens here.

Thanks - and thanks to Matthew Wilcox too.   Fixed.

> 
> > diff --git a/kernel/task_work.c b/kernel/task_work.c
> > index 95a7e1b7f1da..aec19876e121 100644
> > --- a/kernel/task_work.c
> > +++ b/kernel/task_work.c
> > @@ -183,3 +183,4 @@ void task_work_run(void)
> >  		} while (work);
> >  	}
> >  }
> > +EXPORT_SYMBOL(task_work_run);
> 
> If we're exporting this, then I think that function needs a big
> disclaimer on exactly when it is safe to call it. And it most certainly
> needs to be a _GPL export.

I've added

 * Can be used by a kernel thread but only when no locks are held and the
 * thread won't be waited for by other code that might hold locks.  It
 * can be useful in the top-level loop of a file-serving thread to ensure
 * files get closed promptly.

to the documentation comment.
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.

But if there are other strong opinions, or clearly documented rules that
contradict my opinion, I have not problem with adding _GPL.

Thanks,
NeilBrown

> 
> -- 
> Jens Axboe
> 
>