linux-kernel - Re: linux-next: Tree for July 18: warning at kernel/lockdep.c:2068 trace_hardirqs_on

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Date:	Sun, 20 Jul 2008 00:27:26 +0200
From:	"Vegard Nossum" <vegard.nossum@...il.com>
To:	"Greg KH" <gregkh@...e.de>
Cc:	"Mariusz Kozlowski" <m.kozlowski@...land.pl>,
	"Dave Hansen" <haveblue@...ibm.com>,
	"Stephen Rothwell" <sfr@...b.auug.org.au>,
	kernel-testers@...r.kernel.org, linux-next@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>,
	"Pekka Enberg" <penberg@...helsinki.fi>
Subject: Re: linux-next: Tree for July 18: warning at kernel/lockdep.c:2068 trace_hardirqs_on_caller

On Sun, Jul 20, 2008 at 12:17 AM, Greg KH <gregkh@...e.de> wrote:
> On Sat, Jul 19, 2008 at 02:59:12PM +0200, Vegard Nossum wrote:
>> On Sat, Jul 19, 2008 at 11:55 AM, Vegard Nossum <vegard.nossum@...il.com> wrote:
>> > What I don't get here is how SLUB can be used this early in the boot
>> > process. Notice that this is still miles away from the
>> >
>> >    SLUB: Genslabs=12, HWalign=128, Order=0-3, MinObjects=0, CPUs=1, Nodes=1
>> >
>> > line, which comes much later. And that kobject_init() _is_ calling
>> > kzalloc() via verify_dynamic_kobject_allocation(). Isn't this an
>> > error?
>> >
>> > (Unfortunately, my "git log" doesn't turn up any recent changes for
>> > any of the affected code paths here.)
>>
>> Ehe... and this is the reason why: The code was added by this patch:
>>
>> commit 0e3638d1e04040121af00195f7e4628078246489
>> Author: Dave Hansen <haveblue@...ibm.com>
>> Date:   Thu Mar 16 17:30:16 2006 -0800
>>
>>     warn when statically-allocated kobjects are used
>>
>> ..which only exists in -next. Is that just a truly ancient patch, or
>> did somebody forget to adjust their clock?
>
> It is truely a very old patch, that only lives in my tree, and currently
> isn't planned to go to Linus any year soon.
>
> But it has a very long history of living in the -mm tree, and finding
> real bugs, it's just not "safe" enough to go to Linus's tree.  Unless
> you think it is?

Hm. In this case, the patch is not even reporting a problem, it is in
fact in error itself.

The problem is that it calls kzalloc() before the slab caches have
been set up. (Yes, it's a wonder that nothing crashed.) I can only
suggest the addendum

    if (!slab_is_available())
        return;


Vegard

-- 
"The animistic metaphor of the bug that maliciously sneaked in while
the programmer was not looking is intellectually dishonest as it
disguises that the error is the programmer's own creation."
	-- E. W. Dijkstra, EWD1036
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