On Fri, Feb 22, 2013 at 01:08:17PM -0800, Linus Torvalds wrote: > On Fri, Feb 22, 2013 at 7:06 AM, Thomas Gleixner <t...@linutronix.de> wrote: > >> > >> 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... > > > > plus the butt ugly softirq semantics or the lack thereof ... > > The softirq semantics are perfectly fine. Don't blame softirq for the > fact that irq_exit() has had shit-for-brains for a long time. > > Just move the whole "invoke_softirq()" thing down to *after* the > tick_nohz_irq_exit() stuff. And that "wakeup_softirqd()" is garbage > too, since the whole thing should only be used for the > "force_irqthreads" case (which invoke_softirq()" got right.
The issue here is that softirqs may change some timers state (ie: enqueue new timer list, dequeue, modify...), so tick_nohz_irq_exit() really need to be called after irq tail time softirq processing in order to propagate the timer changes to the hardware. But tick_nohz_irq_exit() may trigger the timer softirq itself. Another issue is rcu_irq_exit(). softirqs may make use of RCU read side critical sections, so rcu_irq_exit() has to be called after softirq processing. Meanwhile it seems that RCU may raise its softirq from rcu_irq_exit(). Perhaps Paul could confirm that. So we have a circular dependency problem here. I guess the sanest solution is to rework tick_nohz_irq_exit() and rcu_irq_exit() to remove the use of softirqs there. And even trigger a warning if that happens. Or we need some way to force raise through ksoftirqd but that doesn't sound like a proper long term solution. > And get rid of that final > > #ifndef __ARCH_IRQ_EXIT_IRQS_DISABLED > local_irq_restore(flags); > #endif > > because even if the architecture enters irq_exit() with interrupts > enabled, we should damn well exit with them disabled so that there are > no races with new recursive interrupts (other than the ones that > wakeup_softirqd already handled). Agreed, we always return from irq_exit() with irqs disabled as long as we processed pending softirqs anyway. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
