Messages in this thread | | | Date | Sat, 21 Sep 2013 09:47:26 +0200 | From | Ingo Molnar <> | Subject | Re: [RFC GIT PULL] softirq: Consolidation and stack overrun fix |
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* Linus Torvalds <torvalds@linux-foundation.org> wrote:
> On Fri, Sep 20, 2013 at 9:26 AM, Frederic Weisbecker <fweisbec@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. > > 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. > > - 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. > > Hmm?
I'd definitely argue in favor of never letting unknown-size stacks nest (i.e. to always switch if we start a new context on top of a non-trivial stack).
Known (small) size stack nesting is not real stack nesting, it's just a somewhat unusual (and faster) way of stack switching.
Thanks,
Ingo
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