Re: start_kernel(): bug: interrupts were enabled early

From: David Howells
Date: Fri Apr 02 2010 - 10:47:42 EST

Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> 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

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