Re: lockdep and threaded IRQs (was: ...)

Ingo Molnar
2009-03-03T09:46:15Z

* Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:

> On Mon, 2009-03-02 at 18:37 -0800, David Brownell wrote:
> > No. But I did get a non-response that didn't include any
> > explanation, and relied totally on unfounded assertions
> > combined with the presumption that someday IRQF_DISABLED
> > will be forced on in all drivers.
>
> Enabling IRQs in hardirq context is BAD because:
>
> - IRQ handler nesting leads to stack overflow
> - It gives the false impression its OK for IRQ handlers to be slow,
> it is _NOT_, as you still generate horrible preemption latency.
>
> Therefore IRQF_DISABLED _will_ be forced on everybody some day
> soon, and I'll provide an IRQF_ENABLED for use by broken
> hardware only (and make a TAINT flag for that too).

Basically the problem why !IRQF_DISABLED is bad that if there
are enough interrupt handlers we can get nesting like this:

<irq 20>
<handler runs with irqs enabled>
<irq 21>
<handler runs with irqs enabled>
<irq 22>
<handler runs with irqs enabled>
<irq 23>
<handler runs with irqs enabled>
<irq 24>
<handler runs with irqs enabled>

Suppose each handler gets interruped while it already used up
1000 bytes of the stack (conservative estimation - often it's
more) - the above sequence is already 5000 bytes into the stack.

There is no protection against stack overflow there and such
bugs can be _very_ hard to trigger and find. If there's a
sufficient number of devices and a high enough load it can
trigger spuriously.

Yes, in a few limited embedded environments where you dont have
more than 3-4 IRQ sources you might decide that it's safe to do
(or you might decide that you dont care). Also, there's a few
legacy pieces of hardware with either very long hw access
latencies or too short buffers. Plus there might be any number
of other hw factors - or architecture details (such as the use
of separate per IRQ stacks) that limit IRQ handler parallelism
in practice.

So we'll have the quirk flag for the weird cases - but these are
the exceptions that strengthen the general rule. The concept of
enabling interrupts in a hardirq handler is a no-no on a general
purpose kernel and no modern driver should make use of it.

I hope this explains why lockdep never supported this case.

Ingo