linux-kernel - Re: start_kernel(): bug: interrupts were enabled early

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Date:	Thu, 1 Apr 2010 13:12:36 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	"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, David Howells <dhowells@...hat.com>
Subject: Re: start_kernel(): bug: interrupts were enabled early



On Thu, 1 Apr 2010, Andrew Morton wrote:
>
> > So making the slow-path do the spin_[un]lock_irq{save,restore}() versions 
> > sounds like the right thing. It won't be a performance issue: it _is_ the 
> > slow-path, and we're already doing the expensive part (the spinlock itself 
> > and the irq thing).
> 
> It's actually on the fastpath for lib/rwsem-spinlock.c.

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.

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.

				Linus
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