bug seen with dynticks from CONFIG_HARDIRQS_SW_RESEND

[Woodruff, Richard]

Hi,

In testing we were noticing that we were getting some intermittent
crashes in profile_tick() when dyntick was enabled.

The crashes were because the frame pointer per_cpu____irq_regs value was
0.  That code does a user_mode(get_irq_regs()).  Currently regs is set
only upon real hardware entry on an irq.

The crash path shows resend_irqs() could be called with in a context
where set_irq_regs() was not executed.  In one specific case this was
from
softirq->tasklet_action(resend_tasklet)->resend_irqs->handle_level_irq->
handle_IRQ_event->...->profile_tick.

It seems anyone calling kernel/irq/manage.c:enable_irq() at the wrong
time can trigger this crash.

Creating a fake stack and doing a set_irq_regs() fixes the crash.  Would
it be useful to set a pointer to the entry context on all state changes?
For ease I just hacked a default fake stack into the init process after
fork time so there is never a 0 but that doesn't seem so nice.

Regards,
Richard W.

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