We have always done that (since long before 1.0). The nesting is a
per-interrupt thing, and is only disallowed for the _same_ interrupt.
Also note that if all interrupt handlers on a given IRQ line have the
SA_INTERRUPT flag set, then 2.1.x (and all earlier kernels for that
matter) don't allow any nesting at all, because the irq handler won't
enable interrupt locally. You can still get "nested" interrupts (of a
different type) on different CPU's, but they'll also have different
stacks.
I wonder if the "nesting" Alan was talking about was nesting a timer
interrupt (or something else) with another interrupt. That has always been
allowed, and still is. And that is still disallowed by "fast" interrupts
(it is in fact the only meaning that the SA_INTERRUPT flag still has).
Linus
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