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Date:	Fri, 02 Apr 2010 15:46:14 +0100
From:	David Howells <dhowells@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	dhowells@...hat.com, Andrew Morton <akpm@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>, Yinghai Lu <yinghai@...nel.org>,
	Rabin Vincent <rabin@....in>,
	lkml <linux-kernel@...r.kernel.org>, penberg@...helsinki.fi,
	cl@...ux-foundation.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	linux-arch@...r.kernel.org
Subject: Re: start_kernel(): bug: interrupts were enabled early

Linus Torvalds <torvalds@...ux-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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