linux-kernel - Re: [PATCH 2/9] timers: provide a "modern" variant of timers

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Message-ID: <20170521182312.GZ390@ZenIV.linux.org.uk>
Date:   Sun, 21 May 2017 19:23:12 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Christoph Hellwig <hch@....de>, Arnd Bergmann <arnd@...db.de>,
        Mark Gross <mark.gross@...el.com>, Tejun Heo <tj@...nel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/9] timers: provide a "modern" variant of timers

On Sun, May 21, 2017 at 07:57:53PM +0200, Thomas Gleixner wrote:
> On Thu, 18 May 2017, Christoph Hellwig wrote:
> > On Thu, May 18, 2017 at 10:24:48AM +0200, Christoph Hellwig wrote:
> > > > b) give the union a name (breaks any reference to timer_list->func in C code):
> > > > 
> > > >  +       union {
> > > >  +               void            (*func)(struct timer_list *timer);
> > > >  +               void            (*function)(unsigned long);
> > > >  +       } u;
> > > 
> > > I'll look into that, as it seems a lot safer, and places outside
> > > the timer code shouldn't really touch it (although I bet they do,
> > > so more fixes for this series..)
> > 
> > Meh.  All the old init_timer users set function directly, so
> > I guess we need to use the other approach.
> 
> There is another possibility. Create a coccinelle script which wraps all
> 
>       timer.function = f;
>       timer->function = f;
> 
> assignements into a helper timer_set_function(timer, func) and ask Linus to
> run it right before the next -rc. That handles everything in tree and the
> few new instances in next can be addressed with patches sent to the
> maintainers.

FWIW, there was another possible approach - I toyed with that several years
ago, but it didn't go anywhere.  Namely, make timer.function take void *
*and* turn the setup part into setup(timer, callback, argument), verifying
that
	* callback(argument) will be acceptable expression for C typechecking
	* callback returns void
	* argument is a pointer type
then cast callback to void (*)(void *) and argument to void *.  That way
we get rid of any boilerplate in callbacks and get sane typechecking...