2013/9/20 Linus Torvalds <torva...@linux-foundation.org>: > On Fri, Sep 20, 2013 at 9:26 AM, Frederic Weisbecker <fweis...@gmail.com> > wrote: >> >> Now just for clarity, what do we then do with inline sofirq executions: on >> local_bh_enable() >> for example, or explicit calls to do_softirq() other than irq exit? > > If we do a softirq because it was pending and we did a > "local_bh_enable()" in normal code, we need a new stack. The > "local_bh_enable()" may be pretty deep in the callchain on a normal > process stack, so I think it would be safest to switch to a separate > stack for softirq handling.
Right. > > So you have a few different cases: > > - irq_exit(). The irq stack is by definition empty (assuming > itq_exit() is done on the irq stack), so doing softirq in that context > should be fine. However, that assumes that if we get *another* > interrupt, then we'll switch stacks again, so this does mean that we > need two irq stacks. No, irq's don't nest, but if we run softirq on > the first irq stack, the other irq *can* nest that softirq. Well, most archs don't define __ARCH_IRQ_EXIT_IRQS_DISABLED. It doesn't even mean that the majority of them actually run irq_exit() with irqs enabled in practice. But there may be thoretically some where hardirqs can nest without even the help of softirqs. So it's quite possible to run softirqs on a hardirq stack that is not empty. Now certainly what needs to be fixed then is archs that don't have __ARCH_IRQ_EXIT_IRQS_DISABLED or archs that have any other significant opportunity to nest interrupt. > > - process context doing local_bh_enable, and a bh became pending > while it was disabled. See above: this needs a stack switch. Which > stack to use is open, again assuming that a hardirq coming in will > switch to yet another stack. Right. Now if we do like Thomas suggested, we can have a common irq stack that is big enough for hard and softirqs. After all there should never be more than two or three nesting irq contexts: hardirq->softirq->hardirq, softirq->hardirq, ... At least if we put aside the unsane archs that can nest irqs somehow. -- 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/