linux-kernel - Re: [PATCH 05/12] mm: alloc_contig

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Date:	Thu, 31 Mar 2011 21:52:26 +0200
From:	"Michal Nazarewicz" <mina86@...a86.com>
To:	"Steven Rostedt" <rostedt@...dmis.org>
Cc:	"Marek Szyprowski" <m.szyprowski@...sung.com>,
	"Dave Hansen" <dave@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-samsung-soc@...r.kernel.org, linux-media@...r.kernel.org,
	linux-mm@...ck.org, "Kyungmin Park" <kyungmin.park@...sung.com>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>,
	"Ankita Garg" <ankita@...ibm.com>,
	"Daniel Walker" <dwalker@...eaurora.org>,
	"Johan MOSSBERG" <johan.xx.mossberg@...ricsson.com>,
	"Mel Gorman" <mel@....ul.ie>, "Pawel Osciak" <pawel@...iak.com>
Subject: Re: [PATCH 05/12] mm: alloc_contig_range() added

On Thu, 31 Mar 2011 21:28:21 +0200, Steven Rostedt wrote:
> WARN_ON() should never do anything but test. That ret++ does not belong
> inside the WARN_ON() condition. If there are other locations in the
> kernel that do that, then those locations need to be fixed.

Testing implies evaluating, so if we allow:

     if (++i == end) { /* ... */ }

I see no reason why not to allow:

     if (WARN_ON(++i == end)) { /* ... */ }

In both cases the condition is tested.

>> On Thu, 2011-03-31 at 15:16 +0200, Marek Szyprowski wrote:
>>> +       ret = 0;
>>> +       while (!PageBuddy(pfn_to_page(start & (~0UL << ret))))
>>> +               if (WARN_ON(++ret >= MAX_ORDER))
>>> +                       return -EINVAL;

> On Thu, Mar 31, 2011 at 09:02:41AM -0700, Dave Hansen wrote:
>> In any case, please pull the ++ret bit out of the WARN_ON().  Some
>> people like to do:
>>
>> #define WARN_ON(...) do{}while(0)
>>
>> to save space on some systems.

On Thu, 31 Mar 2011 21:26:50 +0200, Steven Rostedt wrote:
> That should be fixed, as the if (WARN_ON()) has become a standard in
> most of the kernel. Removing WARN_ON() should be:
>
> #define WARN_ON(x) ({0;})

This would break a lot of code which expect that testing to take place.
Also see <http://lxr.linux.no/linux+*/include/asm-generic/bug.h#L108>.

> But I agree, that there should be no "side effects" inside a WARN_ON(),
> which that "++ret" is definitely one.

Thus I don't really agree with this point.

At any rate, I don't really care.

-- 
Best regards,                                         _     _
.o. | Liege of Serenely Enlightened Majesty of      o' \,=./ `o
..o | Computer Science,  Michal "mina86" Nazarewicz    (o o)
ooo +-----<email/xmpp: mnazarewicz@...gle.com>-----ooO--(_)--Ooo--
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