backport patches to 2.6.34 to remove __ARCH_WANT_INTERRUPTS_ON_CTXSW?

[Li Zefan]

Hi Catalin,

We got system crashes, and then we managed to trigger the bug within minutes,
and we found this in upstream, which also backported to 2.6.34 stable:

commit cb297a3e433dbdcf7ad81e0564e7b804c941ff0d
Author: Chanho Min <chanho0207 at gmail.com>
Date:   Thu Jan 5 20:00:19 2012 +0900

    sched/rt: Fix task stack corruption under __ARCH_WANT_INTERRUPTS_ON_CTXSW

The bug described in this commit resembles to ours. Unfortunately After applying
the fix, we still get crash in hours. We tried to bind each real-time task to a
single cpu to make sure no cpu migration will happen, and it ran without any
problem for ~20 hours.

We're still investigating this issue. One thing I'm doing is backporting patches
that removes __ARCH_WANT_INTERRUPTS_ON_CTXSW. With those patches, I can boot
the kernel, but it hung up when the system automatically start nfs and later
soft-lockup was reported. Things are fine if I disable nfs startup and start it
manually.

So did I miss something when backporting, or is it infeasible to backport them
to 2.6.34? We're using ARMv7. I've attached the patches I backported.

Thanks,
Li Zefan
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