Re: [PATCH 2/2] irq: Cleanup context state transitions in irq_exit()

From: Frederic Weisbecker
Date: Fri Feb 22 2013 - 10:04:58 EST

On Fri, Feb 22, 2013 at 03:33:51PM +0100, Thomas Gleixner wrote:
> On Fri, 22 Feb 2013, Frederic Weisbecker wrote:
> > The irq code is run under HARDIRQ_OFFSET preempt offset until
> > we reach the softirq code. Then it's substracted, leaving the
> > preempt count to 0, whether we have pending softirqs or not.
> >
> > Afterward, if we have softirqs to run, we'll run them under
> > the SOFTIRQ_OFFSET then set the preempt offset back to 0
> > after softirqs completion.
> >
> > The rest of the code in irq_exit(), mainly tick_nohz_irq_exit()
> > and rcu_irq_exit(), are executed with this NULL preempt offset.
> >
> > The semantics here are not very intuitive. They leave several portions
> > of the code into some half-defined context state, where in_interrupt()
> > returns false while we actually are in an interrupt.
> >
> > In order to make the context definition less confusing, let's
> > cover the whole code in irq_exit() under HARDIRQ_OFFSET, except
> > for the softirq processing where we switch back and forth
> > from HARDIRQ_OFFSET to SOFTIRQ_OFFSET without leaving a gap
> > in the context definition.
> >
> > There is a drawback though: raise_softirq() relies on the previous
> > semantics considering that as long as we are in_interrupt(), the
> > pending softirq will be handled in the end of the interrupt. Otherwise
> > it kicks ksoftirqd so the softirq is always processed.
> >
> > Now tick_nohz_irq_exit() and rcu_irq_exit() can raise softirqs
> > themselves. Since these functions were not under the HARDIRQ_OFFSET,
> > calling raise_softirq() resulted in waking up the ksoftirqd thread.
> > This is correct because invoke_softirq() has already been invoked at
> > this stage. But with this patch they are now under HARDIRQ_OFFSET and
> > raise_softirq() wrongly thinks invoke_softirq() has yet to be called.
> > In the end, this could leave the softirq unprocessed for a while.
> >
> > So as Thomas suggested me, this also brings a check in the end of
> > irq_exit() that looks for pending softirqs raised after invoke_softirq()
> > and wake up ksoftirqd if needed.
> >
> > Given that the cleanup on the contexts transition involves that
> > new unpretty workaround, I have mixed feelings about this patch so I
> > tagged it as "RFC" and I wait for your opinion.
> Of course, I'm all for it because I suggested that solution :)
> Seriously, I prefer to have in_interrupt() and in_irq() working in the
> functions which are called from irq_exit() rather than having special
> case workarounds inside of them. We are in interrupt context at this
> point and not in some magic virtual place.
> The minimal extra check at the end of irq_exit() is way better than
> any other special cased workaround and the softirq stuff is really the
> only thing which needs to be taken care of. Anything else just works.

Yeah that's indeed a point in favour of this patch.

I prefer to let you guys have the final word on this patch. Whether you
apply it or not, I fear I'll never be entirely happy either way :)
That's the sad fate of dealing with circular dependencies...
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