linux-kernel - Re: [patch 04/11] posix-timers: Remove pointless unlock

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Message-ID: <20250224215555.GF11590@noisy.programming.kicks-ass.net>
Date: Mon, 24 Feb 2025 22:55:55 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	Frederic Weisbecker <frederic@...nel.org>,
	Benjamin Segall <bsegall@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Andrey Vagin <avagin@...nvz.org>,
	Pavel Tikhomirov <ptikhomirov@...tuozzo.com>
Subject: Re: [patch 04/11] posix-timers: Remove pointless unlock_timer()
 wrapper

On Mon, Feb 24, 2025 at 07:43:26PM +0100, Thomas Gleixner wrote:
> On Mon, Feb 24 2025 at 17:21, Peter Zijlstra wrote:
> > On Mon, Feb 24, 2025 at 11:15:28AM +0100, Thomas Gleixner wrote:
> >> It's just a wrapper around spin_unlock_irqrestore() with zero value.
> >
> > Well, I disagree... the value is that is matches lock_timer(). Both in
> > naming and in argument types.
> 
> Sure, but it's not used consistently as we have places where
> lock_timer() is not involved.
> 
> > @@ -327,14 +350,13 @@ bool posixtimer_deliver_signal(struct ke
> >  	 * Release siglock to ensure proper locking order versus
> >  	 * timr::it_lock. Keep interrupts disabled.
> >  	 */
> > -	spin_unlock(&current->sighand->siglock);
> > +	guard(spinlock)(&current->sighand->siglock);
> 
> How is that equivalent?

I R idiot :-)

> So the resulting code is:
> 
> 	scoped_guard (lock_timer, timer_id) {
> 		struct k_itimer *timr = __guard_ptr(lock_timer)(&scope);
> 		const struct k_clock *kc;
> 
> 		memset(setting, 0, sizeof(*setting));
> 		kc = timr->kclock;
> 		if (WARN_ON_ONCE(!kc || !kc->timer_get))
> 			return -EINVAL;
>  
> 		return 0;
> 	}
> 	return -EINVAL;
> 
> I had to go and stare at the guard/class muck 10 times to convince
> myself, that this actually works. This really wants to be express the
> condition of the scoped_guard() somehow, e.g. scoped_cond_guard() or
> such.

Right, so the alternative form is something like:

	scoped_cond_guard (lock_timer, return -EINVAL, timer_id) {
		struct k_itimer *timr = __guard_ptr(lock_timer)(&scope);
		const struct k_clock *kc;

		memset(setting, 0, sizeof(*setting));
		kc = timr->kclock;
		if (WARN_ON_ONCE(!kc || !kc->timer_get))
			return -EINVAL;
	}
	return 0;

Is that really so much better?

> >  /* Delete a POSIX.1b interval timer. */
> >  SYSCALL_DEFINE1(timer_delete, timer_t, timer_id)
> >  {
> > -	return posix_timer_delete(NULL, timer_id);
> > +	scoped_guard (lock_timer, timer_id) {
> > +		posix_timer_invalidate(scope.lock, scope.flags);
> > +		scoped_guard_end(lock_timer);
> > +		posix_timer_unhash_and_free(scope.lock);
> 
> Not sure whether it's a good idea to free the scope.lock and not
> scope.timer :)

There is no scope.timer, the way this work is that the main pointer is
.lock, per the __DEFINE_UNLOCK_GUARD() helper.

I said there were rough edges :-/

Anyway, should I continue poking at this to see if I can clean it up /
extract more useful helpers.

Or shall I just let it be.