linux-kernel - Re: [tip:core/urgent] WARN_ON_SMP(): Allow use in if() statements on UP

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Message-ID: <20110325195643.GB7346@elte.hu>
Date:	Fri, 25 Mar 2011 20:56:43 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Alexey Dobriyan <adobriyan@...il.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	dvhart@...ux.intel.com, peterz@...radead.org,
	akpm@...ux-foundation.org, srostedt@...hat.com, tglx@...utronix.de,
	laijs@...fujitsu.com, linux-tip-commits@...r.kernel.org
Subject: Re: [tip:core/urgent] WARN_ON_SMP(): Allow use in if() statements on
 UP


* Alexey Dobriyan <adobriyan@...il.com> wrote:

> On Fri, Mar 25, 2011 at 03:40:34PM -0400, Steven Rostedt wrote:
> > On Fri, 2011-03-25 at 21:31 +0200, Alexey Dobriyan wrote:
> > 
> > > > Would you like me to add a comment that states:
> > > > 
> > > > 	/*
> > > > 	 * Use ({0;}) as just "0" will cause gcc to output:
> > > > 	 *   warning: statement with no effect
> > > > 	 */
> > > 
> > > Try ((void)0)
> > 
> > Maybe I should update the comment to stop further confusion:
> 
> Ah!
> 
> I always thought if (WARN_ON(x)) is confusing and should be banned.

I'd encourage you to read kernel/lockdep.c one day:

		return DEBUG_LOCKS_WARN_ON(1);
		DEBUG_LOCKS_WARN_ON(1);
	if (DEBUG_LOCKS_WARN_ON(class->subclass != subclass))
	if (DEBUG_LOCKS_WARN_ON(!irqs_disabled()))
		if (DEBUG_LOCKS_WARN_ON(id >= MAX_LOCKDEP_KEYS))
	if (DEBUG_LOCKS_WARN_ON(unlikely(early_boot_irqs_disabled)))
	if (DEBUG_LOCKS_WARN_ON(!irqs_disabled()))
	if (DEBUG_LOCKS_WARN_ON(current->hardirq_context))
	if (DEBUG_LOCKS_WARN_ON(!irqs_disabled()))
	if (DEBUG_LOCKS_WARN_ON(!irqs_disabled()))
	if (DEBUG_LOCKS_WARN_ON(!irqs_disabled()))
		DEBUG_LOCKS_WARN_ON(!softirq_count());
	if (DEBUG_LOCKS_WARN_ON(irqs_disabled_flags(flags)))
	if (DEBUG_LOCKS_WARN_ON(!name)) {
	if (DEBUG_LOCKS_WARN_ON(!key))
		DEBUG_LOCKS_WARN_ON(1);
	if (DEBUG_LOCKS_WARN_ON(!irqs_disabled()))
	if (DEBUG_LOCKS_WARN_ON(depth >= MAX_LOCK_DEPTH))
	if (DEBUG_LOCKS_WARN_ON(!class))
	if (DEBUG_LOCKS_WARN_ON(id >= MAX_LOCKDEP_KEYS))
		if (DEBUG_LOCKS_WARN_ON(chain_key != 0))
	if (DEBUG_LOCKS_WARN_ON(!irqs_disabled()))
		if (DEBUG_LOCKS_WARN_ON(!class))
		if (DEBUG_LOCKS_WARN_ON(!hlock->nest_lock))
	if (DEBUG_LOCKS_WARN_ON(!depth))
	if (DEBUG_LOCKS_WARN_ON(curr->lockdep_depth != depth))
	if (DEBUG_LOCKS_WARN_ON(!depth))
	if (DEBUG_LOCKS_WARN_ON(curr->lockdep_depth != depth - 1))
	if (DEBUG_LOCKS_WARN_ON(!depth && (hlock->prev_chain_key != 0)))
		if (DEBUG_LOCKS_WARN_ON(current->hardirqs_enabled)) {
		if (DEBUG_LOCKS_WARN_ON(!current->hardirqs_enabled)) {
			DEBUG_LOCKS_WARN_ON(current->softirqs_enabled);
			DEBUG_LOCKS_WARN_ON(!current->softirqs_enabled);
	if (DEBUG_LOCKS_WARN_ON(!depth))
	if (DEBUG_LOCKS_WARN_ON(!depth))

These conditional warnings made the flow a lot clearer and simpler - while 
still giving us a very robust lockdep machinery that wont crash no matter what 
kind of anomaly happens.

But yes, i can imagine such constructs being misused as well when mixed into 
real, non-debug functionality. As usual, it's just a tool, with good and evil 
uses as well - so your 'it should be banned' position is too extreme IMHO.

Thanks,

	Ingo
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