Unset LOCKDEP and TRACE_IRQFLAGS_SUPPORT

[Valdis.Kletnieks at vt.edu]

On Fri, 02 Oct 2015 18:11:53 +0200, Gilles Chanteperdrix said:

> TRACE_IRQFLAGS is broken with the I-pipe patch. At least on ARM. The
> visible result is LOCKDEP false positives, but there may be some
> more subtle kernel data structures corruption due to the fact that
> the TRACE_IRQFLAFGS code is called from the Xenomai context. The
> principle of Xenomai is to be able to run while Linux believes
> its interrupts are off, in the middle of its critical sections. For
> this to be possible, the code run in this context must never touch
> any Linux kernel data structures. Enabling TRACE_IRQFLAGS breaks
> this assumption: Linux code which potentially accesses Linux data
> structures gets called from the Xenomai context. The consequences
> can be anything, such as mlock failing, though in practice we never
> get any report about that. The usual reason for mlockall failing is
> because the user is not root, and with default ulimit settings at
> least, mlockall can not be called by non-root users.

Thanks for the detailed explanation - It's a refreshing change from the
usual cargo-cult programming "It seems to break but we have no idea why,
so just turn it off" we often see with out-of-tree patches.

One thing jumped out at me:

> The principle of Xenomai is to be able to run while Linux believes
> its interrupts are off, in the middle of its critical sections

Are you saying that it enables interrupts when Linux thinks they are off?

If so, all I can say is: You're a far braver man than I. :)
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