[PATCH v2] arm: Fix backtrace generation when IPI is masked

On Tue, Sep 15, 2015 at 12:05:12PM +0100, Daniel Thompson wrote:

Currently on ARM when <SysRq-L> is triggered from an interrupt handler
(e.g. a SysRq issued using UART or kbd) the main CPU will wedge for ten
seconds with interrupts masked before issuing a backtrace for every CPU
except itself.

The new backtrace code introduced by commit 96f0e00378d4 ("ARM: add
basic support for on-demand backtrace of other CPUs") does not work
correctly when run from an interrupt handler because IPI_CPU_BACKTRACE
is used to generate the backtrace on all CPUs but cannot preempt the
current calling context.

This patch needs a little more work - what happens to the IPI_CPU_BACKTRACE
we've sent to ourselves?  (It fires after the interrupt handler for the
UART/kbd has finished.)  It ought to be masked out if we're going to
handle it a different way.

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