Re: [announce, patch, RFC] "big IRQ lock" removal, IRQ cleanups.

[george anzinger]

Oleg Nesterov wrote:
> 
> Hello.
> 
> > - to remove the preemption count increase/decrease code from the lowlevel
> >   IRQ assembly code.

IMHO this is unwise as the system NEEDS to exit to user (or
system) space when preempt_count is zero with a garuntee
that TIF_NEED_RESCHED is 0.  To do this from irq.c means
that it must exit with interrupts off and the the low level
code needs to keep them off till the irtn.  But this is
where we test TIF_NEED_RESCHED, and if it is set, schedule()
is called and should be called with preempt_count != 0, to
avoid stack overflow interrupt loops.  And, schedule()
returns with the interrupt system on (even if we, unwisely,
call it with it off).  
> 
> So do_IRQ() can start with preempt_count == 0.
> Suppose another cpu sets TIF_NEED_RESCHED flag
> at the same time.
> 
> spin_lock(&desc->lock) sets preempt_count == 1.
> Before calling handle_IRQ_event() (which adds IRQ_OFFSET
> to preempt_count), do_IRQ() does spin_unlock(&desc->lock)
> and falls into schedule().
> 
> Am I missed something?

Nope, you got it right.
> 
> It seems to me that call to irq_enter() must be shifted
> from handle_IRQ_event() to do_IRQ().

Even then...
> 
> Oleg.
> -
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-- 
George Anzinger   [e-mail]
High-res-timers: 
http://sourceforge.net/projects/high-res-timers/
Real time sched:  http://sourceforge.net/projects/rtsched/
Preemption patch:
http://www.kernel.org/pub/linux/kernel/people/rml