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    SubjectRe: start_kernel(): bug: interrupts were enabled early
    Date
    Linus Torvalds <torvalds@linux-foundation.org> wrote:

    > Ahh, yes. In this case, that doesn't likely change anything. The
    > save/restore versions of the irq-safe locks shouldn't be appreciably more
    > expensive than the non-saving ones. And architectures that really care
    > should have done their own per-arch optimized version anyway.

    That depends on the CPU. Some CPUs have quite expensive interrupt disablement
    instructions. FRV does for instance; but fortunately, on the FRV, I can use
    some of the excessive quantities of conditional registers to pretend that I
    disable interrupts, and only actually do so if an interrupt actually happens.

    > Maybe we should even document that - so that nobody else makes the mistake
    > x86-64 did of thinking that the "generic spinlock" version of the rwsem's
    > is anything but a hacky and bad fallback case.

    In some cases, it's actually the best way. On a UP machine, for instance,
    where they reduce to nothing or where your only atomic instruction is an XCHG
    equivalent.

    David


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