Message-ID: <50A11B3B.1080600@oracle.com>
Date: Mon, 12 Nov 2012 10:52:27 -0500
From: Sasha Levin <sasha.levin@oracle.com>
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To: Russell King - ARM Linux <linux@arm.linux.org.uk>
CC: Maarten Lankhorst <m.b.lankhorst@gmail.com>, linux-kernel@vger.kernel.org,
        Kukjin Kim <kgene.kim@samsung.com>,
        linux-arm-kernel@lists.infradead.org,
        linux-samsung-soc@vger.kernel.org
Subject: Re: [PATCH] ARM: EXYNOS: use BUG_ON where possible
References: <1352406191-14303-1-git-send-email-sasha.levin@oracle.com> <1352406191-14303-5-git-send-email-sasha.levin@oracle.com> <50A111DD.6080504@gmail.com> <20121112152349.GH28327@n2100.arm.linux.org.uk>
In-Reply-To: <20121112152349.GH28327@n2100.arm.linux.org.uk>
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On 11/12/2012 10:23 AM, Russell King - ARM Linux wrote:
> On Mon, Nov 12, 2012 at 04:12:29PM +0100, Maarten Lankhorst wrote:
>> Op 08-11-12 21:23, Sasha Levin schreef:
>>> @@ -465,10 +465,8 @@ static void __init combiner_cascade_irq(unsigned int combiner_nr, unsigned int i
>>>  	else
>>>  		max_nr = EXYNOS4_MAX_COMBINER_NR;
>>>  
>>> -	if (combiner_nr >= max_nr)
>>> -		BUG();
>>> -	if (irq_set_handler_data(irq, &combiner_data[combiner_nr]) != 0)
>>> -		BUG();
>>> +	BUG_ON(combiner_nr >= max_nr);
>>> +	BUG_ON(irq_set_handler_data(irq, &combiner_data[combiner_nr]) != 0);
>>
>> Is it really a good idea to put functions that perform work in a BUG_ON?
>> I don't know, but for some reason it just feels wrong. I'd expect code to
>> compile fine if BUG_ON was a noop, so doing verification calls only, not
>> actual work..
> 
> Well, it is currently defined as:
> 
> include/asm-generic/bug.h:#define BUG_ON(condition) do { if (unlikely(condition)) BUG(); } while(0)
> include/asm-generic/bug.h:#define BUG_ON(condition) do { if (condition) ; } while(0)
> 
> but as these can be overridden, I don't think relying on those
> implementations is a good idea; to do so would be fragile.  Eg, what if
> the BUG_ON() implementation becomes just:
> 
> #define BUG_ON(x)
> 
> then the function call itself vanishes.  So, only put the actual bug test
> inside a BUG_ON(), not the functional part which must always be executed.

Even if we ignore that modifying the side-effects is wrong, there's already
more than enough code in the kernel (both in kernel/ / mm/, and in arch/) to
cause breakage if for some reason the expression is not evaluated.

If some arch decides to not evaluate the expression there it's going to be
inherently broken.


Thanks,
Sasha


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