Re: start_kernel(): bug: interrupts were enabled early
From: Linus Torvalds
Date: Fri Apr 02 2010 - 11:00:47 EST
On Fri, 2 Apr 2010, David Howells wrote:
> 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.
I think you're missing the part where we're not _adding_ any irq disables:
we're just changing the unconditional irq disable to a save-and-disable
(and the unconditional irq enable to a restore).
So even if irq's are expensive to disable, the change from
won't make that code any more expensive.
> > 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
Again, you seem to think that we used to have just a plain spin_lock. Not
so. We currently have a spin_lock_irq(), and it is NOT a no-op even on UP.
It does that irq disable.
Anyway, I suspect that even with just an atomic xchg, you can do a better
job at doing down_read() than using the generic spin-lock version (likely
by busy-looping on a special "we're busy" value). But if you do want to
use the generic spin-lock version, I doubt any architecture makes that
irqsave version noticeable slower than the unconditional irq version.
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