linux-kernel - Re: [2.6.28] NULL pointer dereference at get

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Message-Id: <200812310201.GJH43235.OFFLHFMStOJVOQ@I-love.SAKURA.ne.jp>
Date:	Wed, 31 Dec 2008 02:01:56 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	frederik.deweerdt@...og.eu
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	kosaki.motohiro@...fujitsu.com, netdev@...r.kernel.org
Subject: Re: [2.6.28] NULL pointer dereference at get_stats()

Hello.

Frederik Deweerdt wrote:
> Does adding maxcpus=0 to the boot parameters solve the problem? Stephen
> suspected a per-cpu variable related problem.

No, but I got interestring results.

Result | # of virtual CPUs | Kernel command line
-------+-------------------+-----------------------------------------
 OK    | 2                 | ro root=LABEL=/
 BUG   | 2                 | ro root=LABEL=/ noapic nolapic
 BUG   | 2                 | ro root=LABEL=/ noapic nolapic maxcpus=0
 BUG   | 2                 | ro root=LABEL=/ noapic nolapic nosmp
 BUG   | 2                 | ro root=LABEL=/ maxcpus=0
 BUG   | 2                 | ro root=LABEL=/ nosmp
 OK    | 1                 | ro root=LABEL=/
 OK    | 1                 | ro root=LABEL=/ nolapic noapic
 OK    | 1                 | ro root=LABEL=/ nolapic noapic maxcpus=0
 OK    | 1                 | ro root=LABEL=/ nolapic noapic nosmp

"OK" means NULL pointer dereference didn't happen.
"BUG" means NULL pointer dereference happened.

I was adding "noapic" "nolapic" to command line while assigning 2 vcpus.
Thus, I encountered this problem. Workaround is to remove "noapic" "nolapic".

Now, it seems to me that this problem is caused by interaction of
"noapic" "nolapic" "maxcpus=0" "nosmp" options.

Regards.
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