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

[David Howells <dhowells@redhat.com>]

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