linux-kernel - Re: [patch 6/13] signal/timer/event fds v8

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Date:	Sat, 31 Mar 2007 12:46:58 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch 6/13] signal/timer/event fds v8 - timerfd core ...

On Fri, 30 Mar 2007, Andrew Morton wrote:

> On Fri, 30 Mar 2007 17:47:28 -0700 (PDT) Davide Libenzi <davidel@...ilserver.org> wrote:
> 
> > On Fri, 30 Mar 2007, Andrew Morton wrote:
> > 
> > > > +struct timerfd_ctx {
> > > > +	struct hrtimer tmr;
> > > > +	ktime_t tintv;
> > > > +	spinlock_t lock;
> > > > +	wait_queue_head_t wqh;
> > > > +	unsigned long ticks;
> > > > +};
> > > 
> > > Did you consider using the (presently unused) lock inside wqh instead of
> > > adding a new one?  That's a little bit rude, poking into waitqueue
> > > internals like that, but we do it elsewhere and tricks like that are
> > > acceptable in core-kernel, I guess.
> > 
> > Please, no. Gain is not worth the plug into the structure design IMO.
> > 
> 
> The decision is not that obvious - your patch's main use of
> timerfd_ctx.lock is to provide locking for wqh - ie: to duplicate the
> function of the existing lock which is there for that purpose.
> 
> So I think it's a legitimate optimisation to borrow it.

As I see it, the lock protects the ticks and the timer structure, and the 
wait queue *happen* to have a lock inside. That's the whole purpose in 
having data modeled by structures with accessor functions.
IMO, given that we'd get by plugging into the wait_queue_head_t lock is 
not much (if at all), it'd be better to to peek at the wait_queue_head_t 
directly.




> > > > +static enum hrtimer_restart timerfd_tmrproc(struct hrtimer *htmr)
> > > > +{
> > > > +	struct timerfd_ctx *ctx = container_of(htmr, struct timerfd_ctx, tmr);
> > > > +	enum hrtimer_restart rval = HRTIMER_NORESTART;
> > > > +	unsigned long flags;
> > > > +
> > > > +	spin_lock_irqsave(&ctx->lock, flags);
> > > > +	ctx->ticks++;
> > > > +	wake_up_locked(&ctx->wqh);
> > > > +	if (ctx->tintv.tv64 != 0) {
> > > > +		hrtimer_forward(htmr, hrtimer_cb_get_time(htmr), ctx->tintv);
> > > > +		rval = HRTIMER_RESTART;
> > > > +	}
> > > > +	spin_unlock_irqrestore(&ctx->lock, flags);
> > > > +
> > > > +	return rval;
> > > > +}
> > > 
> > > What's this do?
> > 
> > Really, do we need to comment such trivial code? There is *nothing* that 
> > is worth a line of comment in there. IMO useless comment are more annoying 
> > than blank lines.
> > 
> 
> Look at it from the point of view of someone who knows kernel code but does
> not specifically know this subsystem.  That describes the great majority of
> people who will be reading your code.

Okie, I added a comment ;)



> Patch headers are not maintainable.
> 
> Nobody wants to have to go off and waddle though the git repo to understand
> the design intent behind each function.

I agree. I thought we were talking about the "poor smuck" having to 
document it, and the patch header is a pretty good start (and maybe 
finish ;).



- Davide


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