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SubjectRe: The mysterious case of struct irqaction's mask field.

* Rusty Russell <rusty@rustcorp.com.au> wrote:

> Hi all,
>
> As part of the cpumask conversion, I came across struct irqaction:
>
> struct irqaction {
> irq_handler_t handler;
> unsigned long flags;
> cpumask_t mask;
> ...
> };
>
> Most people have been setting 'mask' to CPU_MASK_NONE, and I wondered if that
> really meant that they never want this action performed on any CPU.
>
> But I couldn't find anyone who actually *reads* the 'mask' field. Tracing
> back, it was converted from an unsigned long to a cpumask_t by wli around
> 2.6.7 ("as it was intended to be"). But that conversion didn't reveal anyone
> actually using the field either.
>
> At one point, sparc64 seems to have overloaded it for some kind of irq bucket
> scheme.
>
> Finally, I tracked it back to the creation of (then per-arch) struct irqaction
> in 1.1.82, and this hunk from linux/arch/i386/kernel/irq.c:
>
> @@ -12,14 +12,7 @@
>
> /*
> * IRQ's are in fact implemented a bit like signal handlers for the kernel.
> - * The same sigaction struct is used, and with similar semantics (ie there
> - * is a SA_INTERRUPT flag etc). Naturally it's not a 1:1 relation, but there
> - * are similarities.
> - *
> - * sa_handler(int irq_NR) is the default function called (0 if no).
> - * sa_mask is horribly ugly (I won't even mention it)
> - * sa_flags contains various info: SA_INTERRUPT etc
> - * sa_restorer is the unused
> + * Naturally it's not a 1:1 relation, but there are similarities.
> */
>
> #include <linux/ptrace.h>
>
> So, it was never a cpumask at all; just a remanent of the use of sigaction for
> interrupt handlers. We've been happily setting it throughout the kernel since
> 1995.

Hehe, nice one :-)

> On the assumption that it has failed to coerce the spirits of our ancestors to
> land among us, I'll create a patch to remove it.

Please do.

This seems to be a classic symptom of write-mostly kernel source code :-/

Ingo


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