linux-kernel - Re: 2.6.26-git: NULL pointer deref in __switch

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Message-ID: <4858CA6B.7060109@trash.net>
Date:	Wed, 18 Jun 2008 10:42:19 +0200
From:	Patrick McHardy <kaber@...sh.net>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
CC:	Simon Holm Thøgersen <odie@...aau.dk>,
	Vegard Nossum <vegard.nossum@...il.com>,
	Linux Kernel Mailinglist <linux-kernel@...r.kernel.org>,
	Chuck Ebbert <cebbert@...hat.com>,
	"x86@...nel.org" <x86@...nel.org>, rusty@...tcorp.com.au
Subject: Re: 2.6.26-git: NULL pointer deref in __switch_to

Suresh Siddha wrote:
> Simon, Patrick, I am able to reproduce the oops in __switch_to()
> with lguest.  My debug showed that there is atleast one lguest specific
> issue (which should be present in 2.6.25 and before aswell) and it got
> exposed with a kernel oops with the recent fpu dynamic allocation patches.
>
> In addition to the previous possible scenario (with fpu_counter), in the
> presence of lguest, it is possible that the cpu's TS bit it still set and the
> lguest launcher task's thread_info has TS_USEDFPU still set.
>
> This is because of the way the lguest launcher handling the guest's TS bit.
> (look at lguest_set_ts() in lguest_arch_run_guest()). This can result
> in a DNA fault while doing unlazy_fpu() in __switch_to(). This will
> end up causing a DNA fault in the context of new process thats
> getting context switched in (as opossed to handling DNA fault in the context
> of lguest launcher/helper process).
>
> This is wrong in both pre and post 2.6.25 kernels. In the recent
> 2.6.26-rc series, this is showing up as NULL pointer dereferences or
> sleeping function called from atomic context(__switch_to()), as
> we free and dynamically allocate the FPU context for the newly
> created threads. Older kernels might show some FPU corruption for processes
> running inside of lguest.
>
> With the appended patch, my test system is running for more than 50 mins
> now. So atleast some of your oops (hopefully all!) should get fixed.
> Please give it a try. I will spend more time with this fix tomorrow.
>
> Apart from the last hunk(MSR_IA32_SYSENTER_CS changes), I believe
> the below patch is needed for 2.6.25 aswell.
>   

The patch seems to have fixed the problem for me, thanks Suresh.


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